Summary: John reveals to us that some will turn a way, what will turn them away and what will prevent us from falling from the truth.
The Doctrines of Devils
1 Timothy 4:1 Now the Spirit expressly says that in later times some will depart from the faith by devoting themselves to deceitful spirits and teachings of demons,
I. Those Who Lead Away (18).
1Jn 2:18 "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."
According to prophecy interpretation and the words of Jesus, John knew that we were living in the last dispensation. Each dispensation served God's purposes, revealing to us His nature, our weaknesses and His perfect redemption plan (Erich Sauer, The Dawn of Redemption). Those messages were understood by the Apostles and they also recognized that after this dispensation, no other would be needed before Christ could reign.
As John said, "it is the last hour"; Last =eschatos, from which we get the subject of end times, eschatology. Hour=hora, which could mean hour, time or season. Obviously, we are in the last times and have been since the resurrection.
2 Thessalonians 2:3ff tells us that the lawless one would come.
"... the son of destruction, who opposes and exalts himself against every so-called god or object of worship, so that he takes his seat in the temple of God, proclaiming himself to be God." 3b-4.
9-12 "The coming of the lawless one is by the activity of Satan with all power and false signs and wonders, and with all wicked deception for those who are perishing, because they refused to love the truth and so be saved. Therefore God sends them a strong delusion, so that they may believe what is false, in order that all may be condemned who did not believe the truth but had pleasure in unrighteousness."
The people got this. "...you have heard..." You get it. Great.
2 Thessalonians 2:7 told them, "For the mystery of lawlessness is already at work. Only he who now restrains it will do so until he is out of the way."
Thus John says, "...so now many antichrists have come. Therefore we know that it is the last hour."
26 "I write these things to you about those who are trying to deceive you."
II. Those Who Turn Away.
1Jn 2:19 "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."
Clearly, John is asserting that there were "many antichrists" that had ALREADY arisen. John states that these ’antichrists’ came from within the Church. More specifically, John states the first person plural, "us", as in "from us" as opposed as "from you". This is an indication that those who promote the spirit of the antichrist can trace their roots to the early apostolic teachings.
In other words, the spirit of the antichrist has found it's home in those off-shoots of Christianity. What are referred to as "Christian Cults" are those which John was warning about, those who have an ounce of truth to sell their pound of corruption. Those are the ones who hold the greatest threat to weak Christians.