It's not a Christian belief that God took Mary to heaven. It's not found in the Bible. It's a belief of Catholicism but Catholicism is not Christianity.

Catholicism teaches that you're saved by infant baptism, works, and obedience to church rules. Christianity teaches that we're saved by faith alone in Christ alone guided by the Bible alone.

She's not the mother of God. She was the mother of Jesus' earthly body. Jesus existed in heaven for eternity before he was born in Bethlehem. She's not holy. Only God is holy.