Warning Concerning Antichrists
18 Children, it is the last hour, and as you have heard that antichrist is coming, so now many antichrists have come. Therefore we know that it is the last hour. 19 They went out from us, but they were not of us; for if they had been of us, they would have continued with us. But they went out, that it might become plain that they all are not of us. 20 But you have been anointed by the Holy One, and you all have knowledge. 21 I write to you, not because you do not know the truth, but because you know it, and because no lie is of the truth. 22 Who is the liar but he who denies that Jesus is the Christ? This is the antichrist, he who denies the Father and the Son. 23 No one who denies the Son has the Father. Whoever confesses the Son has the Father also. 24 Let what you heard from the beginning abide in you. If what you heard from the beginning abides in you, then you too will abide in the Son and in the Father. 25 And this is the promise that he made to us—eternal life.

John is establishing many things in this section. He begins by establishing the fact that many anti-christ’s have come. You may be thinking he is speaking specifically of the “THE” definite article anti-Christ and he is…although he is saying and defining much more as we look closer to the text. In context in verse 18 he does not have the definite article in the orginal text (although it could be anarthrous), he is speaking of the Nominative Masculine SINGULAR antic-Christ. However in his discussion of many anti-Christs (Nominitive, Masculine Plural) he opens a much bigger playing field. He actually explains later in chapter 4:1-3 what I believe he is refuting in the cessationist’s theology: 4 Beloved, do not believe every spirit, but test the spirits to see whether they are from God, for many false prophets have gone out into the world. 2 By this you know the Spirit of God: every spirit that confesses that Jesus Christ has come in the flesh is from God, 3 and every spirit that does not confess Jesus is not from God. This is the spirit of the antichrist, which you heard was coming and now is in the world already.
Also in 2 John 7 he states:
7 For many deceivers have gone out into the world, those who do not confess the coming of Jesus Christ in the flesh. Such a one is the deceiver and the antichrist.

So to be clear, John has not written this to correct the cessationist, but to correct their false teaching. The spirit of the antichrist denies either the humanity(incarnation) of Jesus to the expense of His Diety or the Diety at the expense of His humanity. The cessationist actions proved what camp they were in by their leaving, and their flawed doctrine was revealed in their actions. John will expose their flawed view of the hypostatic union in no uncertain terms. The issue of John writing to his beloved, children so that they may know they are saved (5:13) revolves around a right understanding of the nature of Jesus Christ’s incarnation. So to connect the dots and bring us up to the current context, John has shown that God is light and in Him there is no darkness (1:5). He has given us the connection between the Father and the Son and will continue to explain with precision a right view of Jesus incarnation.

As we move through this section don’t miss out on some of Johns maxims. Once he has explained about the correlation of the spirit of the anti-christ and it was revealed in those who left and didn’t come back he explains that they (the believers, 1Jn 5:13) have all been anointed by the Holy One and they all have knowledge. Not only have the believers been anointed by the Holy One with knowledge, he explains he’s not writing to them because they don’t know the truth but because they need to understand THERE IS NO LIE IN THE TRUTH. The inerrancy and the authority of Scripture is without question. The believers need to understand that truth and lies don’t intermingle. This is revealed in the issue of those who leave. As Akin rightly surmises:

John has put forward the test of obedience and the test of love, and now he adds the test of perseverance. Bruce notes, ‘The perseverance of the saints is a biblical doctrine, but it is not a doctrine designed to lull the indifferent into a sense of false security; it means that perseverance is an essential token of sanctity.’ John is confident that those who belong to God will remain with God and, by extension, with the community of faith. This verse is not meant to scare people into obedience but to strengthen the faith of the Johannine church, which had been torn apart by the actions of the dissenters. Once again it is clear that having one’s name on the church toll does not necessarily mean that one’s name is written in the Book of Life” (The New American Commentary, Daniel Akin, p116)

John goes on to again define the relationship between the Father and the Son (vss 22-23) and exposes more of the dissenter’s bad theology. Notice with the close of this section by default John is explaining that those who have been anointed by the Holy One, who have knowledge that contains no lies, will have a net effect of abiding in the Son and in the Father. What was the promise to those who abide…eternal life.

So the question stands, are we abiding? John has much more to say about what it means to abide as he walks us through the surety of our salvation.