A.T. Robertson Commentary 1 John 3:16

"Hereby know we love, because he laid down his life for us: and we ought to lay down our lives for the brethren." — 1 John 3:16 (ASV)

Know we (εγνωκαμεν). Perfect active indicative, "we have come to know and still know." See 2:3 for "hereby" (εν τουτω).

Love (την αγαπην). "The thing called love" (D. Smith).

He for us (εκεινος υπερ ημων). Εκεινος as in 2:6; 3:3,5, υπερ here alone in this Epistle, though common in John's Gospel (10:11,15; 11:50, etc.) and in 3 John 1:7.

Laid down his life (την ψυχην αυτου εθηκεν). First aorist active indicative of τιθημ, the very idiom used by Jesus of himself in Joh 10:11,17f.

We ought (ημεις οφειλομεν). Emphatic ημεις again. For οφειλω see 2:6. Of course our laying down our lives for the brethren has no atoning value in our cases as in that of Christ, but is a supreme proof of one's love (John 13:37f.; 15:13), as often happens.