The pope only speaks for Catholics, not Christians.
As a Christian myself, I can tell you, there are big differences between Catholicism and Christianity.
For one thing, Christians don't see the Pope as a holy man. They see him as some average joe who thinks he speaks on behalf of god--or basically, that the pope is a false prophet of sorts. We also don't have priests you have to confess to, or rosaries--those are strictly catholic things.