In 1 John 2:19, we read the following: "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."
This verse is often cited as teaching that believers cannot fall away from the faith. However, when read in context, it is easy to see that the Apostle John is not talking about believers in general. Rather, he is talking about a certain group of people (whether they are unbelievers or deceived believers is not clear) who seemed to be of the rank of the apostles but later showed themselves to be false apostles and antichrists in doctrine. John is saying that a true apostle cannot fall away from the apostolic doctrine. In contrast to these false apostles, he addresses mature Spirit-taught believers (verse 13-14) in the immediate verses that follow: "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." By saying this, John is giving his approval to these mature believers, among whom are certainly many elders (verses 13-14) who have leadership roles in the church. It is John's desire that these mature believers stand firm in their faith and not be deceived by these false apostles (whom he refers to as antichrists). He makes this explicit in verse 26 when he says, "These things have I written unto you concerning them that seduce you." If these antichrists had left the church and forsaken fellowship, John would not have needed to warn his readers about them. Rather, they are still in the churches and pose a danger to others.
To reiterate, when John says, "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:", he is not talking about the impossibility of a believer falling away from the faith. Rather, he is saying that it is impossible for a true apostle (i.e., the emphatic "us" in verse 19) to depart from the apostolic doctrine. Furthermore, he wants mature Spirit-taught believers (i.e., the emphatic "you" in verses 20-21) to stand firm in their faith so that these deceivers (verse 26) will not cause harm to anyone. This distinction between "us" (the apostles) and "you" (the mature Spirit-taught believers) was made explicit at the very start of the letter (1 John 1:1-5).
As to whether or not there were any true believers among these antichrists, it is not clear. Certainly, believers can become deceived and teach heresy (Acts 20:30). Jesus even once went so far as to rebuke the Apostle Peter, calling him Satan (Matthew 16:23). In that instance, Peter clearly exhibited the antichrist spirit. The moral of the story is that we believers should always be cautious, "examining [ourselves], whether [we] be in the faith" (2 Corinthians 13:5), "lest [we] also, being led away with the error of the wicked, fall from [our] own stedfastness" (2 Peter 3:17). We cannot lose eternal life, but if we are not careful, we can fall into heresy and sedition (Galatians 5:20).