A.T. Robertson Commentary 1 John 3:24

"And he that keepeth his commandments abideth in him, and he in him. And hereby we know that he abideth in us, by the Spirit which he gave us." — 1 John 3:24 (ASV)

And he in him (κα αυτος εν αυτω). That is "God abides in him" as in 4:15. We abide in God and God abides in us through the Holy Spirit (John 14:10,17,23; 17:21). "Therefore let God be a home to thee, and be thou the home of God: abide in God, and let God abide in thee" (Bede).

By the Spirit (εκ του πνευματος). It is thus (by the Holy Spirit, first mention in this Epistle and "Holy" not used with "Spirit" in this Epistle or the Apocalypse) that we know that God abides in us.

Which (ου). Ablative case by attraction from accusative ο (object of εδωκεν) to agree with πνευματος as often, though not always. It is a pity that the grammatical gender (which) is retained here in the English instead of "whom," as it should be.