Albert Barnes Commentary 1 John 4:3

"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 4:3 (ASV)

And every spirit that confesseth not, etc. That is, this doctrine is essential to the Christian system, and anyone who does not hold it cannot be regarded as a Christian or recognized as a Christian teacher. If he was not a man, then everything that occurred in his life—in Gethsemane and on the cross—was in appearance only and was assumed merely to delude the senses.

There were no real sufferings; there was no shedding of blood; there was no death on the cross; and, consequently, there was no atonement. A mere show, an assumed appearance, a vision, could not make atonement for sin. Therefore, a denial of the doctrine that the Son of God had come in the flesh was, in fact, a denial of the doctrine of expiation for sin.

The Latin Vulgate here reads qui solvit Jesure, "who dissolves or divides Jesus." Socrates (H. E. vii. 32) says that in the old copies of the New Testament it is written, o liei ton ihsoun, "who dissolves or divides Jesus"—that is, who separates his true nature or person, or who supposes that there were two Christs, one in appearance and one in reality.

This reading was found early in some manuscripts and is referred to by many of the Fathers (see Wetstein), but it has no real authority and was evidently introduced, perhaps at first from a marginal note, to oppose the prevailing errors of the times. The common reading, who confesseth not, is found in all the Greek manuscripts, in the Syriac versions, and in the Arabic. As Lucke says, the other reading is manifestly of Latin origin. The common reading in the text is that which is supported by authority and is entirely in accordance with John's manner.

And this is that spirit of anti-christ. This is one of the things that characterize antichrist. John here refers not to an individual who should be known as antichrist, but to a class of persons. This does not, however, forbid the idea that there might be some one individual, or a succession of persons in the church, to whom the name might be applied preeminently (see Barnes on 1 John 2:18).

Compare 2 Thessalonians 2:3 and following (see Barnes).

Therefore you have heard that it should come (see Barnes on 1 John 2:18).