John Gill 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)

And we have known and believed Or have a full assurance and knowledge of, and faith in,

the love that God has to us; shown as in many instances, so more especially in sending his Son to be the propitiation for our sins, to be the Saviour of us, and that we might live through him.

God is love; (See Gill on 1 John 4:8):

and he that dwells in love; who dwells by faith upon the love of God as displayed in Christ, and abides in the exercise of love to God and to the saints:

dwells in God, and God in him; (See Gill on 1 John 4:13); the last clause, "and God in him", is left out in the Syriac version.