The doctrine that infants who die in Original Sin alone go to Hell can be seen very early in Catholic history, such as in the cases of Pope Innocent I (401-417 AD), Pope Gelasius I (492-496 AD), and Pope Gregory I (540-604 AD), who taught such a thing explicitly:
Augustine - Against Two Letters of the Pelagians, Book 2, Chapter 7
Will infants after the end of their life, even if while they live they are not baptized in Christ, be in eternal life, or will they not? If he should say, "They will", how then did he answer that he had condemned what had been uttered under his name "according to the judgment of Innocent, of blessed memory"? Lo, Pope Innocent, of blessed memory, says that infants have not life without Christ's baptism, and without partaking of Christ's body and blood.
Pope Gelasius I - Letter VII, To All the Bishops of Picenum
As for infants, who, according to this heresy, are said not to be condemned for original sin alone, such a proposition is both impious and profane. While every Christian knows that infants, fresh from their mothers' wombs, are baptized for the remission of sins, which the Catholic Church rightly celebrates, it is not done falsely, but truly. For this reason, because they have no personal sins, it is certain that only their original sins are forgiven. Therefore, since all their sins are remitted through baptism, they attain eternal life; it follows that if their sins are not remitted, they cannot attain eternal life.
There is nothing they can say, such as that the unbaptized children can merely be excluded from the Kingdom of Heaven without eternal damnation. Since without baptism they cannot eat the body or drink the blood of Christ, they cannot have life in themselves; therefore, without this life, they will inevitably be condemned to eternal death. Let them take away this so-called third place they have invented for deceiving the innocent, and since we only read of the right and the left, let them not leave the unbaptized on the left side, but allow those who are baptized to be transferred to the right through holy regeneration.
Pope Gregory I - An Exposition on the Book of Blessed Job, Book 9
32 For there be some that are withdrawn from the present light, before they attain to shew forth the good or evil deserts of an active life. And whereas the Sacraments of salvation do not free them from the sin of their birth, at the same time that here they never did aright by their own act; There they are brought to torment. And these have one wound, viz. to be born in corruption, and another, to die in the flesh. But forasmuch as after death there also follows, death eternal, by a secret and righteous judgment 'wounds are multiplied to them without cause.' For they even receive everlasting torments, who never sinned by their own will. And hence it is written, Even the infant of a single day is not pure in His sight upon earth. Hence (Truth) says by His own lips, Except a man be born of water and of the Spirit, he cannot enter into the kingdom of God.
Historically, the Church has understood this to mean that infants who die before being baptized go to a portion of Hell that is without fire, also called Limbo:
Council of Florence (1445 AD)
Also, the souls of those who have incurred no stain of sin whatsoever after baptism, as well as souls who after incurring the stain of sin have been cleansed whether in their bodies or outside their bodies, as was stated above, are straightaway received into heaven and clearly behold the triune God as he is, yet one person more perfectly than another according to the difference of their merits. But the souls of those who depart this life in actual mortal sin, or in original sin alone, go down straightaway to hell to be punished, but with unequal pains.
With regard to children, since the danger of death is often present and the only remedy available to them is the sacrament of baptism by which they are snatched away from the dominion of the devil and adopted as children of God, it admonishes that sacred baptism is not to be deferred for forty or eighty days or any other period of time in accordance with the usage of some people, but it should be conferred as soon as it conveniently can; and if there is imminent danger of death, the child should be baptized straightaway without any delay, even by a lay man or a woman in the form of the church, if there is no priest, as is contained more fully in the decree on the Armenians.
Notice, the teaching of the Ecumenical Council is extremely clear. The fate of the unbaptized who die in "original sin alone" is unambiguously stated to be Hell. Any proposed exception to this would simply be a rejection of what the council says, when it provides absolutely no room for caveats. And, how could a caveat exist? Are any infants "more deserving" of an exception, than others?
Further, the council urges the immediate baptism of infants, because water baptism is "the only remedy available to them" to be kept from "the dominion of the devil".
The doctrine that infants go to Hell, but to a part of it that is without fire, is reiterated by Pope Pius VI (1717-1799 AD):
Pope Pius VI - Auctorem Fidei (1794 AD)
The Punishment of Those Who Die with Original Sin Only
26 The doctrine which rejects as a Pelagian fable, that place of the lower regions (which the faithful generally designate by the name of the limbo of children) in which the souls of those departing with the sole guilt of original sin are punished with the punishment of the condemned, exclusive of the punishment of fire, just as if, by this very fact, that these who remove the punishment of fire introduced that middle place and state free of guilt and of punishment between the kingdom of God and eternal damnation, such as that about which the Pelagians idly talk - false, rash, injurious to Catholic schools.
Above, what is being rejected as "false, rash, injurious to Catholic schools" is the doctrine that infants burn in Hell. Rather, he is affirming that they do in fact go to the "lower regions", i.e., Hell, to be "punished with the punishment of the condemned, exclusive of the punishment of fire", that is, without fire.
Also, Pope Sixtus V (1521-1590 AD) in the Papal Bull Effraenatam, which was to condemn abortion, says:
Pope Sixtus V - Effraenatam (1588 AD)
Noticing that frequently by various Apostolic Constitutions the audacity and daring of most profligate men, who know no restraint, of sinning with license against the commandment "do not kill" was repressed; We who are placed by the Lord in the supreme throne of justice, being counseled by a most just reason, are in part renewing old laws and in part extending them in order to restrain with just punishment the monstrous and atrocious brutality of those who have no fear to kill most cruelly fetuses still hiding in the maternal viscera.
Who will not detest such an abhorrent and evil act, by which are lost not only the bodies but also the souls? Who will not condemn to a most grave punishment the impiety of him who will exclude a soul created in the image of God and for which Our Lord Jesus Christ has shed His precious Blood, and which is capable of eternal happiness and is destined to be in the company of angels, from the blessed vision of God, and who has impeded as much as he could the filling up of heavenly mansions, and has taken away the service to God by His creature?
Above, the Pope clearly says that the "souls" of the infants are "lost", and the abortionist has "excluded a soul" from the "blessed vision of God", i.e., the Beatific Vision. This is the quintessence of Limbo - which in Catholic thought, does not necessarily confer pain, but does involve the deprivation of the Beatific Vision.
Additionally, the Council of Trent (1545-1563 AD) stated clearly that baptism is required for remission of "original sin", and entrance into Heaven, both generally, and specifically when speaking in the context of unbaptized infants:
Council of Trent (1547 AD) - Session 7
2 If any one shall say, that true and natural water is not of necessity for baptism, and, on that account, wrests to some sort of metaphor those words of our Lord Jesus Christ; Except a man be born again of water and them Holy Ghost; let him be anathema.
5 If any one shall say, that baptism is free, that is, not necessary unto salvation; let him be anathema.
Council of Trent (1546 AD) - Session 5
4 If any one denies that infants, newly born from their mothers' wombs, even though they be sprung from baptized parents, are to be baptized; or says that they are baptized indeed for the remission of sins, but that they draw nought of original sin from Adam, which has need to be expiated by the laver of regeneration, for the obtaining life everlasting - whence it follows, as a consequence, that in them the form of baptism, for the remission of sins, is understood to be not true, but false - let him be anathema. For that which the apostle has said, By one man sin entered into the world, and by sin death, and so death passed upon all men in whom all have sinned, is not to be understood otherwise than as the Catholic Church spread everywhere hath always understood it. For, by reason of this rule of faith, from a tradition of the apostles, even infants, who could not as yet in themselves commit any sin, are for this cause truly baptized for the remission of sins, that in them that which they have contracted by generation, may be cleansed away by regeneration. For, unless a man be born again of water and the Holy Ghost, he cannot enter into the kingdom of God.
Above, in yet another Ecumenical Council, the Church is mandating infant baptism on the basis that it is necessary for Salvation.
And, it is again worth stating that no exceptions are provided for in the text, neither would any exceptions be logically possible. In what case would there be an exception? When the baby's parents aren't Christian? Is a Christian baby any more to blame than a non-Christian baby, for not being baptized? Either way, the child is admittedly innocent, having "original sin" only. So, no exception is given, because the doctrine leaves no room for exceptions.
The same message is reiterated in the Catechism of the Council of Trent:
The Catechism of the Council of Trent (1566 AD) - Baptism Of Infants Should Not Be Delayed
The faithful are earnestly to be exhorted to take care that their children be brought to the church, as soon as it can be done with safety, to receive solemn Baptism. Since infant children have no other means of salvation except Baptism, we may easily understand how grievously those persons sin who permit them to remain without the grace of the Sacrament longer than necessity may require, particularly at an age so tender as to be exposed to numberless dangers of death.
Finally, Pope Pius XII (1876-1958 AD) gave an address in which he explicitly stated that infants and the unborn are not able to perform the "act of love sufficient for the adult to obtain sanctifying grace and to supply the lack of baptism":
Pope Pius XII - Address to Midwives on the Nature of Their Profession (1951 AD)
Above all, the state of grace is absolutely necessary at the moment of death without it salvation and supernatural happiness - the beatific vision of God - are impossible. An act of love is sufficient for the adult to obtain sanctifying grace and to supply the lack of baptism; to the still unborn or newly born this way is not open. Therefore, if it is considered that charity to our fellowman obliges us to assist him in the case of necessity, then this obligation is so much the more important and urgent as the good to be obtained or the evil to be avoided is the greater, and in the measure that the needy person is incapable of helping or saving himself with his own powers; and so it is easy to understand the great importance of providing for the baptism of the child deprived of complete reason who finds himself in grave danger or at death's threshold.
Notice above that Pius XII specifically mentions those who are "still unborn" as being subject to this danger, and in need of baptism, as did Pope Sixtus V in Effraenatam, who spoke of "fetuses still hiding in the maternal viscera" being excluded "from the blessed vision of God". This is due to the Church's doctrine that Original Sin is imbued upon a person at conception. Therefore, the Church teaches that not only newborns and infants, but even the unborn go to Limbo. This necessarily means that there are tens of billions of unborn infants in Limbo, as only about 30% of fertilized eggs will result in a live birth. So, for every human ever born, 2 or 3 went straight to Limbo, having perished before being brought to term.
In summary, the doctrine that infants who die without being water baptized go to Limbo has been consistently and infallibly taught throughout the history of the Roman Catholic Church.
In October of 2004, Pope John Paul II (1920-2005 AD) tasked the Church's International Theological Commission with studying the question of the possibility for salvation of unbaptized infants. This study was completed under the pontificate of Pope Benedict XVI (1927-2022 AD), and the Pope approved the document submitted by the commission on January 19, 2007.
This document stated:
The Hope of Salvation for Infants Who Die Without Being Baptized (2007 AD)
102 Our conclusion is that the many factors that we have considered above give serious theological and liturgical grounds for hope that unbaptized infants who die will be saved and enjoy the Beatific Vision. We emphasize that these are reasons for prayerful hope, rather than grounds for sure knowledge.
The above conclusion, promulgated by Pope Benedict XVI, is directly contradictory to the explicit statements covered in the first section of this article, particularly those of the Council of Florence.
Recall that the Council of Florence stated that water baptism is "the only remedy available" to infants to be kept from "the dominion of the devil", and that all who die "in original sin alone, go down straightaway to hell". That is perfectly, unambiguously clear. As were Pope Innocent I, Pope Gelasius I, Pope Gregory I, Pope Pius VI, Pope Sixtus V, Pope Pius XII, and the Council of Trent.
The Catholic Church's failure to dogmatically take a stand for the their historical doctrine of Limbo presents a contradiction, even if Limbo is not explicitly denied. It is the same as if someone said "The sky is blue", repeatedly, for many years, then when challenged, sheepishly said "There is reason to hope that the sky is not blue". It is a different message.
The modern Catechism of the Catholic Church also states a hope for the Salvation unbaptized infants, again contravening historical Roman Catholicism:
Catechism of the Catholic Church 1261
1261 As regards children who have died without Baptism, the Church can only entrust them to the mercy of God, as she does in her funeral rites for them. Indeed, the great mercy of God who desires that all men should be saved, and Jesus' tenderness toward children which caused him to say: "Let the children come to me, do not hinder them," allow us to hope that there is a way of salvation for children who have died without Baptism. All the more urgent is the Church's call not to prevent little children coming to Christ through the gift of holy Baptism.
How can the above not be read as "there is hope that the Church may have been wrong about the necessity of infant baptism and the existence of Limbo for hundreds of years"?
The above, along with the document promulgated by Pope Benedict XVI, is the simple result of kind people who are rightfully appalled at the Church's historical false doctrine, and so resort to dishonestly pretending that the Church's clear statements on the matter can be ignored, or nullified. Their attempt to square a theological circle requires them to start playing with language, and employing other deceptive, dishonest tactics. The Church was not silent on this issue, it was unambiguous, and slammed the door on the "hope" that the modern Catholic Church professes, many hundreds of years ago.
In the past, the Church clearly and repeatedly taught that infants who die without water baptism go to a section of Hell known as Limbo. It was a well-established, authentic tradition of the Church, taught in many different authoritative documents spanning hundreds of years. However, in modern times, the Church has denied that the fate of unbaptized infants is necessarily Limbo, but rather hopes that they too may go to Heaven.
The Church's failure to dogmatically stand behind Limbo, even though Limbo was taught by an Ecumenical Council and multiple Popes, presents another modern contradiction of convenience for the Roman Catholic Church. These are such that come about by the Church wanting to distance itself from doctrines that it definitely held and taught in the past, because such doctrines are horrific to modern sensibilities. See, for instance, the death penalty, criminalizing homosexuality, banning female altar servers, torturing heretics, confining Jews to ghettos, etc.
Eastern Orthodoxy also teaches the false doctrine that unbaptized infants go to Hell:
Synod of Jerusalem (1672 AD) - Decree 16
We believe Holy Baptism, which was instituted by the Lord, and is conferred in the name of the Holy Trinity, to be of the highest necessity. For without it none is able to be saved, as the Lord says, "Whoever is not born of water and of the Spirit, shall in no way enter into the Kingdom of the Heavens." (John 3:5) And, therefore, baptism is necessary even for infants, since they also are subject to original sin, and without Baptism are not able to obtain its remission. Which the Lord showed when he said, not of some only, but simply and absolutely, "Whoever is not born [again]," which is the same as saying, "All that after the coming of Christ the Savior would enter into the Kingdom of the Heavens must be regenerated." And since infants are men, and as such need salvation, needing salvation they need also Baptism. And those that are not regenerated, since they have not received the remission of hereditary sin, are, of necessity, subject to eternal punishment, and consequently cannot without Baptism be saved. So that even infants should, of necessity, be baptized.
The Orthodox Confession of Saint Peter Mogila (1638-1642 AD)
20 This hereditary sin cannot be rooted out or abolished by any repentance whatever, but only by the grace of God, through the work of redemption, wrought by our Lord Jesus Christ, in taking upon him our flesh and pouring out his precious blood. And this is done in the mystery of holy Baptism; and whosoever is not a partaker thereof, such a one remains unabsolved from his sin, and continueth in his guilt, and is liable to the eternal punishment of the divine wrath: As it is said, Verily, verily, I say unto you, that except a man be born of water, and of the Spirit, he cannot enter into the kingdom of heaven.
The Orthodox Confession of Saint Peter Mogila (1638-1642 AD)
103 And this baptism, which is not to be again repeated, is of such power and efficacy, that it is an undoubted seal and pledge of eternal salvation. Now, what the fruits and benefits are of this mystery may be easily apprehended; for, first, it taketh away original sin in infants, and in those of ripe age it taketh away both original and voluntary sin. Secondly, it plainly renews or regenerates the man, and restores him to that state of justice and righteousness wherein he stood before, while undefiled with sin, in the state of innocence;