According to Catholic teaching, any validly baptised person is a Christian, regardless of their beliefs. Baptism is an indelible mark on the soul, like a spiritual membership card that you cannot destroy. Once you join, you can never really leave. Even if you convert to Islam, or become an atheist, you remain (in some spiritual sense) a Christian as long as you live. This is very different from the Protestant understanding of what it means "to be a Christian", which is based on a person's belief and (more importantly) the inner attitude of their heart.