Adam Clarke on 1 John 4:3

Every spirit - Every teacher, that confesseth not Jesus, is not of God - has not been inspired by God. The words εν σαρκι εληλυθοτα, is come in the flesh, are wanting in AB, several others, both the Syriac, the Polyglot Arabic, Ethiopic, Coptic, Armenian, and Vulgate; in Origen, Cyril, Theodoret, Irenaeus, and others. Griesbach has left them out of the text.

Spirit of antichrist - All the opponents of Christ's incarnation, and consequently of his passion, death, and resurrection, and the benefits to be derived from them.

Ye have heard that it should come - See 2Thes 2:7.

Even now already is it in the world - Is working powerfully both among the Jews and Gentiles.