"Children, it is the last hour; and just as you heard that antichrist is coming, even now many antichrists have appeared; from this we know that it is the last hour. They went out from us, but they were not really of us; for if they had been of us, they would have remained with us; but they went out, so that it would be shown that they all are not of us." (1 John 2:18-19)John is remembering the works of his Lord, "And then if anyone says to you, 'Behold, here is the Christ'; or, 'Behold, He is there'; do not believe him; for false Christs and false prophets will arise, and will show signs and wonders, in order to lead astray, if possible, the elect. But take heed; behold, I have told you everything in advance." (Mark 13:21-23) Jesus warned us that, through out time, there will be those who claim to be Him, complete with their own prophets to give testimony to the false Christ. However, we are not to be dissuaded from our firm believe and faith in Christ, Christ has come, He had delivered His message, the message taught to us by His apostles, and he will return one day in a way that all will know and all will see. Christ has come, there is no need for us to look for or to expect another.
What I find interesting is that John is the only apostolic writer to use the term "antichrist" which means, "opposite of Christ" or to "oppose Christ". Often we, or at least I, think of the antichrist as some end-time prophetic figure, and in truth he is. But John also reminds us that antichrist is a spirit that has already invaded the world and has already made itself know through those who have received this spirit. Specifically, John is speaking of those who, "do not acknowledge Jesus Christ as coming in the flesh." (2 John 7) In John's day, several men had gone out with a message that struck at the heart of both the divinity and humanity of Jesus. Men like Marcion, Simon, and Valentinus. Many of them made a distinction between Jesus the man and Jesus being God, separating Christ into two distinct persons. Thus they either made Jesus a super human who was yet not quite divine or a super God who was in no way united with the humanity which He came to save. These men spoke a message that was opposite to that of Christ and they also opposed the will, purpose, and work of Christ upon the Earth.
Many of these heretics got their start in the church. For example, in the case of Simon, when the Apostles went to Samaria to lay hands on the disciples that they might receive the Holy Spirit, Simon was already there, observing what the Apostles were doing and saying. "Now when Simon saw that the Spirit was bestowed through the laying on of the apostles' hands, he offered them money, saying, 'Give this authority to me as well, so that everyone on whom I lay my hands may receive the Holy Spirit.'" (Acts 8:18-19) However, he was turned away by Peter, saying, "May your silver perish with you, because you thought you could obtain the gift of God with money! You have no part or portion in this matter, for your heart is not right before God." (Acts 8:20-21) This Simon would leave the church and become the leader of a destructive heresy know and the Simonites.
John is not saying that all who leave our churches are antichrist and that they are bringing upon themselves the destruction that they justly deserve. There are many reasons people leave the church and its not always the person who leaves who is in the wrong, if there is wrong to be had. However, he is saying that we should not be surprised when heresies arise and are apparently "birthed" out of the church, for even in the church the spirit of Antichrist can reside, for a time, until it is made manifest in the lives of those who embrace its error. There will be those who leave to appose the church and the message and work of Christ. However, we are not to be alarmed, confused, distracted, or deceived. Jesus has warned us that these things should happen but that we should continue in our devotion to Jesus and to His Church. We must remember the words of Jesus, "So if they say to you, 'Behold, He is in the wilderness,' do not go out, or, 'Behold, He is in the inner rooms,' do not believe them. For just as the lightning comes from the east and flashes even to the west, so will the coming of the Son of Man be." (Matt 24:26-27) Let us remain steadfast and not be distracted by those who would seek to oppose Christ.