1. THE TITLE-CHANGER: doesn’t understand the gospel, and therefore never believed.
Jesus explains it here (Matthew 13:19, Luke 8:12)
In this case, becoming an atheist is not a change of theological position but only a change of title. The title “Christian” never fit this person in the first place.
2. THE MID-SEASON QUITTER: believes in Jesus and pursues discipleship for a while. When faced with temptation or suffering he gives up.
Jesus explains it here (Matthew 13:20–22, Luke 8:13)