The reason why many feel that those that fall away were not true Christians is that the Bible speaks about the "elect"; those that were chosen as Christians from before the foundation of the world. If you have been saved, then that is something that you will never lose. This is based on, among other scriptures, John 3:36 which says: " Whoever believes in the Son has eternal life; whoever does not obey the Son shall not see life, but the wrath of God remains on him." The key word is "HAS" eternal life. That leads to the doctrine of eternal security.
Eternal security is something that we never had as JWs. According to WT teaching, someone can be a true blue Jehovah's Witness, but lose it all because of falling away. Christianity teaches that if someone accepts Jesus as their Lord and Savior, that is something that will never be lost.
This leads to the question, what about those that were "saved" but then fell away? 1 John 2:19 supports what PSac has been saying. It says: " They went out from us, but they did not really belong to us. For if they had belonged to us, they would have remained with us; but their going showed that none of them belonged to us."