Hereby know ye (εν τουτω γινωσκετε). Either present active indicative or imperative. The test of "the Spirit of God" (το πνευμα του θεου) here alone in this Epistle, save verse 1. John 4:13. With the clamour of voices then and now this is important. The test (εν τουτω, as in 1. John 3:19) follows.

That Jesus Christ is come in the flesh (Ιησουν Χριστον εν σαρκ εληλυθοτα). The correct text (perfect active participle predicate accusative), not the infinitive (εληλυθενα, B Vg). The predicate participle (see John 9:22 for predicate accusative with ομολογεω) describes Jesus as already come in the flesh (his actual humanity, not a phantom body as the Docetic Gnostics held). See this same idiom in 2. John 1:7 with ερχομενον (coming). A like test is proposed by Paul for confessing the deity of Jesus Christ in 1. Corinthians 12:3 and for the Incarnation and Resurrection of Jesus in Romans 10:6-10.