Jamieson, Fausset and Brown on 1 John 3:24

24. dwelleth in him--The believer dwelleth in Christ.

and he in him--Christ in the believer. Reciprocity. "Thus he returns to the great keynote of the Epistle, abide in Him, with which the former part concluded" (1Jo 2:28).

hereby--herein we (believers) know that he abideth in us, namely, from (the presence in us of) the Spirit "which He hath given us." Thus he prepares, by the mention of the true Spirit, for the transition to the false "spirit," 1Jo 4:1-6; after which he returns again to the subject of love.