Charles Spurgeon Commentary 1 John 4:1-3

"Beloved, believe not every spirit, but prove the spirits, whether they are of God; because many false prophets are gone out into the world. Hereby know ye the Spirit of God: every spirit that confesseth that Jesus Christ is come in the flesh is of God: 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 4:1-3 (ASV)

But try the spirits whether they are of God: because many false prophets are gone out into the world. Hereby know ye the Spirit of God:

Every spirit that confesseth that Jesus Christ is come in the flesh is of God: and every spirit that confesseth not that Jesus Christ is come in the flesh is not of God: If there is any question raised about the Deity and the humanity of Christ, do not listen any longer. When you taste the first morsel of meat from a joint, and you find that it is tainted, there is no necessity for you to eat all the rest to see if it is good; and if any man questions the true Divinity and the real humanity of Christ, have nothing to do with him, and give no heed to what he says, for he "is not of God."