But we Muslims don't believe that Jesus is God. [Last time I said this some Christians go at me like a ton of ballast though]
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Christians believe that Christ, as the Son of God, is God. Somewhat the same as your son is one with you, or would be if you had one.
His existence is from the beginning and prophecy tells of his birth in Bethlehem. Christians believe that, you believe something different, which is your right.