Hereby know ye (en toutōi ginōskete). Either present active indicative or imperative. The test of “the Spirit of God” (to pneuma tou theou) here alone in this Epistle, save 1Jo 4:13. With the clamour of voices then and now this is important. The test (en toutōi, as in 1Jo 3:19) follows.
That Jesus Christ is come in the flesh (Iēsoun Christon en sarki elēluthota). The correct text (perfect active participle predicate accusative), not the infinitive (elēluthenai, B Vg). The predicate participle (see Joh 9:22 for predicate accusative with homologeō) 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 2Jo 1:7 with erchomenon (coming). A like test is proposed by Paul for confessing the deity of Jesus Christ in 1Co 12:3 and for the Incarnation and Resurrection of Jesus in Rom 10:6-10.