A.T. Robertson Commentary 1 John 4:3

"and every spirit that confesseth not Jesus is not of God: and this is the [spirit] of the antichrist, whereof ye have heard that it cometh; and now it is in the world already." — 1 John 4:3 (ASV)

Confesseth not (μη ομολογε). Indefinite relative clause with the subjective negative μη rather than the usual objective negative ου (verse 6). It is seen also in 2 Peter 1:9; Titus 1:11, a survival of the literary construction (Moulton, Prolegomena, p. 171). The Vulgate (along with Irenaeus, Tertullian, Augustine) reads solvit (λυε) instead of μη ομολογε, which means "separates Jesus," apparently an allusion to the Cerinthian heresy (distinction between Jesus and Christ) as the clause before refers to the Docetic heresy. Many MSS. have here also εν σαρκ εληλυθοτα repeated from preceding clause, but not A B Vg Cop. and not genuine.

The spirit of the antichrist (το του αντιχριστου). Πνευμα (spirit) not expressed, but clearly implied by the neuter singular article to. It is a repetition of the point about antichrists made in 2:18-25.

Whereof (ο). Accusative of person (grammatical neuter referring to πνευμα) with ακουω along with accusative of the thing (οτ ερχετα, as in 2:18, futuristic present middle indicative). Here the perfect active indicative (ακηκοατε), while in 2:18 the aorist (ηκουσατε).

And now already (κα νυν ηδη). As in 2:18 also (many have come). "The prophecy had found fulfilment before the Church had looked for it" (Westcott). It is often so. For ηδη see Joh 4:35; 9:27.