Adam Clarke on 1 John 4:1

Beloved, believe not every spirit - Do not be forward to believe every teacher to be a man sent of God. As in those early times every teacher professed to be inspired by the Spirit of God, because all the prophets had come thus accredited, the term spirit was used to express the man who pretended to be and teach under the Spirit's influence. See 1Cor 12:1-12; 1Tim 4:1.

Try the Spirits - Δοκιμαζετε τα πνευματα· Put these teachers to the proof. Try them by that testimony which is known to have come from the Spirit of God, the word of revelation already given.

Many false prophets - Teachers not inspired by the Spirit of God, are gone out into the world - among the Jewish people particularly, and among them who are carnal and have not the Spirit.