How do you know?
Because he's a Catholic. He's head of the Catholic Church. Not all Christians are Catholic. To liberals, like yourself, the two terms might be interchangable. In the real world, to real Christians the Pope's word has meaning only to Catholics. Other denominations pretty much ignore him. The Pope speaks for and represents a single Christian denomination.
Then you acknowledge that being anti-abortion or against same sex marriage are not Christian positions, IOW,
a person can be pro-choice and pro same sex marriage and be just as Christian as the next person?
A person that is pro-choice and/or pro same sex marriage may call themselves a Christian, but they are not following the teachings of Jesus Christ.