1 John 4:15-16
15 Whosoever shall confess that Jesus is the Son of God, God dwelleth in him, and he in God.
16 And we have known and believed the love that God hath to us. God is love; and he that dwelleth in love dwelleth in God, and God in him.
The Grace New Testament Commentary - 1 John 4:15-16
4:15 "Fellowship" was with the apostles in what they had "seen" (v 14). But the readers were also to have "fellowship" in what the apostles had "heard" (cf. 1 John 1:3). What the readers can hear in the midst of the loving Christian community is nothing less than a confession that Jesus is the Son of God.
The words of John the Baptist that he had seen and testified (John 1:32-34) reflect the wording of the previous verse (we have seen and testify, v 14) whereas the Baptist's words that this is the Son of God reflect the present verse. Thus the "testimony" mentioned in v 14 is not to be limited to the visible manifestation of eternal life in the form of Christian love, although this is a part of it. But John is thinking of a congregational context where there would also regularly be the confession of Jesus as God's Son. John had in mind the visible manifestation of Christian love, accompanied by this confession of Jesus and reproduced in what the apostles themselves had seen in Jesus and what they had heard about Him from His forerunner, John the Baptist. The apostle John's goal of leading his readers into this kind of fellowship with the apostolic circle (1 John 1:1-3) has now been reached.
Furthermore the goal of obtaining an abiding relationship with God is also reached since God abides in anyone who confesses that Jesus is the Son of God, and he in God. This confession is by the Spirit of God by whom the indwelling of God is known and recognized (cf. v 13).
When Jesus is confessed as the Son of God, He is being confessed as "Christ come in flesh" (cf. 1 John 4:2) and as the Guarantor of eternal life and future resurrection to every believer. It is possible for a Christian community to recognize the reality that God "abides" within them if (1) they love one another and (2) they confess that Jesus is the Son of God. Thereby that community is keeping "His commandment" (1 John 3:23).
4:16 Verse 16 is parallel in grammar and theme to v 14. The author apparently intends that both verses show the results of the community experience of love he has been discussing. But between the two verses John has pointed out the role of confessing Jesus (v 15) as an additional sign that God indwells the members of a loving Christian community. Therefore when believers in a church enjoy such an experience as this, they face the reality of the love God has toward them. The statement abides in God and God [abides] in him forms an inclusio with the statement "abides in Him, and He in him" in 1 John 3:24.