Antichrist means instead-of-Christ. Anyone who proposes a means of knowing God that is not through Christ Jesus our Lord is an antichrist.Wider context:
1 John 2:18-23 KJV - 8 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. 19 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.
20 But ye have an unction from the Holy One, and ye know all things.
21 I have not written unto you because ye know not the truth, but because ye know it, and that no lie is of the truth.
22 Who is a liar but he that denieth that Jesus is the Christ? He is antichrist, that denieth the Father and the Son.
23 Whosoever denieth the Son, the same hath not the Father: (but) he that acknowledgeth the Son hath the Father also.
This is about antichrist belief of those who went out from them..
Who denies the Son and hath not the Father.
On first look it's of those who never believed.
It sounds like some joined the church but believing the idea that Jesus Christ was one of many different esoteric teachers who could teach how to enter God-consciousness, and were trying to sell Jesus' methods as merely a re-telling of many other proposed methods. Or perhaps they began relying fully on Jesus, but became persuaded that He was really just one of many teachers of ways to know God, and began to focus on the doctrine instead of the Doctor. When the Jesus-followers refused to trust in the method only, independently of Jesus of Nazareth, the Christ, the frustrated gnostics left to find more gullible audiences. They were not of us means they were not of those who have Jesus Christ as the only way to know communion with God. If they had been of us, relying on Jesus first as the ony way, they would have felt no need to leave out narrow-minded community