What is the spirit of antichrist?
The spirit of antichrist is the spiritual force, active since the time of Christ, that opposes Him and denies that Jesus Christ has come in the flesh; it works in individual hearts and in the church now, and will one day be fully embodied in a man, the Antichrist or Man of Perdition, in the last days.
Scripture calls this the spirit of antichrist, and John defines it plainly: "every spirit that confesseth not that Jesus Christ is come in the flesh is not of God: and this is that spirit of antichrist, whereof ye have heard that it should come; and even now already is it in the world" (1 John 4:3). It is not merely a future figure but a present reality that has been active since the earliest days of the church. This same spirit moved upon Judas, the man of perdition who betrayed Jesus, and it moved the rulers of that day to crucify Him. It continued on to inspire the slaughter of the early church, and it crept into the church itself through preachers who developed false doctrines that eventually caused a falling away.
John warns that even now there are many antichrists in the world, forerunners of the one who is to come: "as ye have heard that antichrist shall come, even now are there many antichrists; whereby we know that it is the last time" (1 John 2:18). Those who walk in this spirit may remain among believers for a season, but they will eventually be revealed and separated out, "they went out from us, but they were not of us" (1 John 2:19). This spirit can also show itself within the heart of a believer, not only through outright denial of Christ, but through unforgiveness, judgmentalism, pride, using past hurts as an excuse not to walk in the Spirit, or receiving another Jesus, another spirit, or another gospel than the one first preached, as Paul warned the Corinthians (2 Corinthians 11:3-4). To hold oneself up as the standard of righteousness and to judge others harshly is, in that sense, to act as a false Christ or antichrist rather than reflecting the true, forgiving nature of Christ.
This spirit of antichrist will be fully and finally manifested as a man in the last days, the Antichrist, the Beast, the Man of Perdition, the son of perdition who is revealed after a falling away (2 Thessalonians 2:3-4). He will make war with the saints and overcome them for a time, being given power over every kindred, tongue, and nation (Revelation 13:7), and he will speak great words against the Most High and wear out the saints for a time, times, and the dividing of time (Daniel 7:25). So the spirit of antichrist works in two ways: as an ongoing spiritual power already present in the world and even within the household of faith, and as the coming embodiment of that same spirit in one final, personal Antichrist who will persecute believers before being broken without hand.
The scriptures
And every spirit that confesseth not that Jesus Christ is come in the flesh is not of God: and this is that spirit of antichrist, whereof ye have heard that it should come; and even now already is it in the world.
Little children, it is the last time: and as ye have heard that antichrist shall come, even now are there many antichrists; whereby we know that it is the last time. They went out from us, but they were not of us; for if they had been of us, they would no doubt have continued with us: but they went out, that they might be made manifest that they were not all of us.
For all that is in the world, the lust of the flesh, and the lust of the eyes, and the pride of life, is not of the Father, but is of the world. And the world passeth away, and the lust thereof: but he that doeth the will of God abideth for ever.
While I was with them in the world, I kept them in thy name: those that thou gavest me I have kept, and none of them is lost, but the son of perdition; that the scripture might be fulfilled.
And after the sop Satan entered into him. Then said Jesus unto him, That thou doest, do quickly.
But I fear, lest by any means, as the serpent beguiled Eve through his subtilty, so your minds should be corrupted from the simplicity that is in Christ. For if he that cometh preacheth another Jesus, whom we have not preached, or if ye receive another spirit, which ye have not received, or another gospel, which ye have not accepted, ye might well bear with him.
Let no man deceive you by any means: for that day shall not come, except there come a falling away first, and that man of sin be revealed, the son of perdition; Who opposeth and exalteth himself above all that is called God, or that is worshipped; so that he as God sitteth in the temple of God, shewing himself that he is God.
And it was given unto him to make war with the saints, and to overcome them: and power was given him over all kindreds, and tongues, and nations.
From the books
This answer is drawn from these passages in the Mary Banks Faith Library.