‘Christianity’ cannot be true because Christianity contains conflicting claims.
When someone says Christianity is true, what they mean is that a specific set of beliefs, ideas, practices, and so on – out of the many that form Christianity – is true, useful, and so on.
Similarly, when someone says that God exists, they mean a specific conception of God.
So, if you say that Christianity is true, you are saying a specific conception of Christianity is true.
Also see here.