A.T. Robertson Commentary 1 John 4:13

"hereby we know that we abide in him and he in us, because he hath given us of his Spirit." — 1 John 4:13 (ASV)

Hereby know we (εν τουτω γινωσκομεν). The Christian's consciousness of the fact of God dwelling in him is due to the Spirit of God whom God has given (δεδωκεν, perfect active indicative here, though the aorist εδωκεν in 3:24). This gift of God is proof of our fellowship with God.