The central motif of Jesus in the Gospel of Matthew is arguably the imminent "Kingdom of Heaven". Its announcement is called "the Gospel of the Kingdom", and that message receives continuous attention (Matthew 3:1-2, 4:17, 4:23, 9:35, 10:7, 13:19, 24:14). Jesus teaches about the Kingdom constantly (Matthew 4:17, 4:23, 5:3, 5:10, 5:19-20, 6:33, 7:21, 8:11-12, 9:35, 10:7, 11:11-12, 12:25-28, 16:19, 16:28, 18:1-4, 19:12-14, 19:23-24, 21:31, 21:43, 23:13, 24:14, 25:34, 26:29). It is featured prominently in the prayer that he teaches to his disciples (Matthew 6:9-13). It is often the focus of his parables (Matthew 13:1-52, 18:23-35, 20:1-16, 22:1-14, 25:1-30). And, five times, he explicitly makes statements which indicate that it is coming very soon:
However, contrary to Jesus's claims, the Kingdom of Heaven did not come within his generation. The "Son of Man" did not "come in the glory of his Father with his angels", and "reward every man according to his works" (Matthew 16:27). Neither did "all the tribes of the earth mourn" because they saw "the Son of man coming in the clouds of heaven with power and great glory" (Matthew 24:30). Neither did God's angels "gather together his elect from the four winds, from one end of heaven to the other" (Matthew 24:31). The main prophecy of Jesus's ministry failed to come to pass as he had predicted.
Note that in referring to himself as the "Son of Man" throughout his preaching, Jesus is claiming to be a specific figure mentioned in the book of Daniel, of which the following is said:
Daniel 7:13-14
13 I saw in the night visions, and, behold, one like the Son of man came with the clouds of heaven, and came to the Ancient of days, and they brought him near before him.
14 And there was given him dominion, and glory, and a kingdom, that all people, nations, and languages, should serve him: his dominion is an everlasting dominion, which shall not pass away, and his kingdom that which shall not be destroyed.
This description of a "Kingdom" given to this "Son of Man" in Daniel inspired/informed basically all of Jesus's preaching in the Synoptic Gospels, and he is made to quote it at the culmination of his ministry (Matthew 26:64, Mark 14:62). It is the most significant passage in the Old Testament from this perspective. However, unfortunately for Jesus, not only is the book of Daniel probably about 300 years younger than he assumed, it also contains a blatant forgery spanning the length of an entire chapter, as covered in this article. Arguably, the book of Daniel inspired Christianity, and yet, it is a forgery. Therefore, it is little surprise that Jesus's prophecies regarding an imminent Kingdom of Heaven based on that book also failed.
An overwhelmingly popular response from Christian apologists to the above presentation would be to state that in "some sense", the Kingdom has already come, and this sense was what was in view in all of the statements of Jesus regarding an imminent Kingdom. "Jesus rules today through the Church". And, why should this surprise anyone? After all, he said:
Luke 17:20-21
20 And when he was demanded of the Pharisees, when the kingdom of God should come, he answered them and said, The kingdom of God cometh not with observation:
21 Neither shall they say, Lo here! or, lo there! for, behold, the kingdom of God is within you.
So there - that takes care of the problem. The Kingdom is spiritual first, and physical later. All of these promises will be literally fulfilled, at the Second Advent of Christ. But, until then, they are spiritually fulfilled, as Christians with the Holy Spirit of God work throughout the world.
Luke 17:21's statement that "the Kingdom of God is within you" is often cited in this discussion, essentially because it is obscure. It is thrown out because it gives plausibility to the idea that Jesus's statements about the Kingdom could have a non-literal meaning.
However, there are some issues with using the statement this way:
So, this passage in Luke 17 is not the rescuing device that some Christians attempt to use it as. Using one passage, rendered in a controversial way, to interpret dozens of others (even in books where the passage is absent) is basically the opposite of how one is supposed to handle a text like the Bible.
Another passage often hauled out in this discussion is a statement in the Gospel of John, where Jesus says:
John 18:36
36 Jesus answered, "My kingdom is not of this world. If My kingdom were of this world, My servants would fight, so that I should not be delivered to the Jews; but now My kingdom is not from here."
Here, Jesus clearly says "My kingdom is not of this world". So, of course it is spiritual/heavenly, at least at first.
However, not only is this passage missing from the Synoptics, and not only is it in what can be called the Forged Gospel, it is really only emphasizing John's theme of contrasting the world from Heaven (John 3:12-13, 3:31, 8:23, etc.). For instance, John's Jesus says that his disciples "are not of the world" (John 15:19, 17:14, 17:16). In other words, Jesus is stating that his Kingdom is from Heaven, rather than rooted in this world, which in John's theology, is associated with evil (1 John 2:15-17, 5:19, etc.). But, since virtually nothing John's Jesus says is reliable, and since this is nowhere in the Synoptics, which actually contain the Kingdom passages, this passage is inadmissible in this discussion. Why not give us a response from Matthew? Because there is no response in Matthew. Matthew gives every indication that the Kingdom was a literal, imminent Kingdom, coming in the first century.
Every response of this sort which attempts to spiritualize or mystify the Kingdom can be shown to be uncompelling by simply re-reading the passages about the Kingdom in the Gospels. The thing that Jesus described as coming within his generation, as mentioned already, included miraculous apocalyptic events (e.g. Matthew 24:29-31), along with an explicit warning that it would be highly visible, and to not listen to anyone who says it could come about in a clandestine way (Matthew 24:26-27).
Another popular response to the Kingdom passages is to state that the Kingdom was offered to Israel, and would have come in the first century if they had accepted Jesus. But, because they crucified him instead, the Kingdom passages have lost at least part of their original meaning/context. Essentially, due to the crucifixion, all bets are off for any timeline that Jesus had established up to that point in his ministry.
However, the glaring, disqualifying problem with this response is that Matthew's Jesus is not surprised by the crucifixion. In fact, many times before he preaches about the coming Kingdom, and before his execution, he discusses/alludes to his crucifixion (Matthew 12:40, 16:21, 17:11-12, 17:22-23, 20:17-19, 20:22-23, 20:28, 21:33-46, 26:1-2). In spite of these passages, Jesus in Matthew 24 still swears that the coming will take place in "this generation" (Matthew 24:34), when specifically asked, "what will be the sign of your coming, and of the end of the age?" (Matthew 24:3).
The Gospel of Matthew was probably written by someone in the latter half of the first century who was expecting these prophecies regarding an imminent Kingdom of Heaven to come true. Yet, they did not. But, because the Synoptic Gospels had become an extremely important source for the life of Jesus by the time that it became clear that the prophecies had failed, they remained in the canon, despite containing these passages which disqualify Jesus as a prophet. Interestingly, John's Gospel completely omits these prophecies, and all of Jesus's public heralding of the Kingdom of Heaven, despite it being arguably the main theme of the Synoptics.
The Christian responses to these failed prophecies do not stand up to scrutiny. Any critic examining Christian responses will notice that almost no one attempts to take all of these passages together, and formulate a comprehensive response. Rather, they are usually kept piecemeal, and apologists attempt to deal with one at a time. That is because when they are put together, their message is clear, and it is also clear that what they are predicting did not happen.