A Christian is saved, but you can fall away from Christ. If this were not true, a multitude of Bible tests would not make sense. Scripture includes many passages that warn believers about falling away. Here two examples from Galatians where Paul is talking to Christians.

Galatians 6:7, 8 "Do not be deceived: God cannot be mocked. A man reaps what he sows. Whoever sows to please their flesh, from the flesh will reap destruction."

Galatians 5: 19-21 "The acts of the flesh are obvious: sexual immorality, impurity and debauchery; 20 idolatry and witchcraft; hatred, discord, jealousy, fits of rage, selfish ambition, dissensions, factions 21 and envy; drunkenness, orgies, and the like. I warn you, as I did before, that those who live like this will not inherit the kingdom of God."

Simply put - Christians have a relationship with God. Like all relationships they can be ignored to the point where you indeed, no longer have a relationship. We are free moral agents - God gives us a choice to continue our walk with Him or to leave the relationship (choose another master).