Christians: Why do you believe in the trinity?

O People of the Book(Christians)! Commit no excesses in your religion: Nor say of God aught but the truth. Christ Jesus the son of Mary was (no more than) a messenger of God, and His Word, which He bestowed on Mary, and a spirit proceeding from Him: so believe in God and His messengers. Say not "Trinity" : desist: it will be better for you: for God is one God: Glory be to Him: (far exalted is He) above having a son. To Him belong all things in the heavens and on earth. And enough is God as a Disposer of affairs. -Quran