Question: "Is there such a thing as an ex-Christian?"
Answer: This is a question for which there is definitely a clear and explicit biblical answer. First John 2:19 declares, "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." This Scripture makes it abundantly clear—there is no such thing as an ex-Christian. If a person is truly a Christian, he/she will never depart from the faith: "for if they had belonged to us, they would have remained with us." If a person who claimed to be a Christian denies the faith, he/she was not truly a Christian. "They went out from us, but they did not really belong to us—their going showed that none of them belonged to us." No, there is truly no such thing as an ex-Christian.
It is important to distinguish between a true Christian and an "in name only" Christian. A true Christian is a person who has fully trusted in Jesus Christ alone for salvation. A true Christian is a person who understands what the Bible says about sin, sin's penalty, who Jesus is, what Jesus did for us, and how that provides for the forgiveness of sin. A true Christian is a person who has received Jesus Christ as personal Savior, has been made a new creation (2 Corinthians 5:17), and is progressively being transformed into the image of Christ. A true Christian is a person who is kept a Christian by the power of the Holy Spirit (Ephesians 4:13, 30; 2 Corinthians 1:22). This true Christian can never become an ex-Christian. No one who has truly and fully trusted in Christ as Savior could ever deny Him. No one who truly comprehends the evil of sin, the terror of sin's consequences, the love of Christ, and the grace and mercy of God, could ever turn back from the Christian faith.
There are many in this world who claim to be Christians, but are not. Being a Christian does not mean being an American or having white skin. Being a Christian does not mean recognizing that Jesus was a great teacher or even seeking to follow His teachings. Being a Christian means being a representative of Christ and a follower / servant of Christ. There are people who have had some connection to a "Christian" church and then later renounced that connection. There are people who have "tasted" and "sampled" Jesus Christ, without ever actually receiving Him as Savior. However, there is no such thing as a true ex-Christian. A true Christian will never, and could never, renounce the faith. Any person who claimed to be a Christian, but later rejects the Christian faith, was never truly a Christian.