A.T. Robertson Commentary 1 John 4:16

"And we know and have believed the love which God hath in us. God is love; and he that abideth in love abideth in God, and God abideth in him." — 1 John 4:16 (ASV)

We know (εγνωκαμεν). Perfect active indicative, "we have come to know and still know" as in Joh 6:9, only there order is changed (πεπιστευκαμεν coming before εγνωκαμεν). Confession (ομολογεω) follows experimental knowledge (γινωσκω) and confident trust (πιστευω). Believers are the sphere (εν ημιν, in our case) in which the love of God operates (Westcott). See Joh 13:35 for "having love."

God is love (ο θεος αγαπη εστιν). Repeated from verse 8. So he gathers up the whole argument that one who is abiding in love is abiding in God and shows that God is abiding in him. Thoroughly Johannine style.