Why are we Christians if Jesus was Jew? I know for you may be a dummy question but I've never understood why are we Christians if Jesus, the God's son, was of Jewish descent?
The genealogy of Jesus Christ, documented both by Matthew and Luke in their gospel accounts shows that he was born of Mary and was descended from both Abraham, the father of all the Hebrews, and descended from David the principal King of Israel.

The whole history of the Old Testament records God's dealings with Israel and his unfolding purpose which was initially demonstrated by him within the confines of one people - Israel - but was always intended to reach farther and to encompass the whole world in due time.

Jesus Christ came of Israel, was born of an Israelite, Mary, was counted as the son of David by being adopted by Joseph, but God's purpose in sending his Son is evidently a purpose that reaches to every one on earth.

Those who believe on Jesus Christ do not revert to that which was, initially, a more restricted situation as God's purposes were intimated and exampled in Jewish rituals and ceremonies. What is now revealed is the reality of a spiritual revelation that far transcends the earthly figures and patterns.