I will correct you because you are wrong.
The Trinity was already well established by the time that the Quran, and the verses permitting marriage to Christians, was revealed.
The Quran (4:171) says, in a verse addressed to the Ahl-e-Kitaab, the same Ahl-e-Kitaab who Muslims are permitted to marry into, that:
O People of the Book! Commit no excesses in your religion: Nor say of Allah aught but the truth. Christ Jesus the son of Mary was (no more than) a messenger of Allah, and His Word, which He bestowed on Mary, and a spirit proceeding from Him: so believe in Allah and His messengers. Say not “Trinity” : desist: it will be better for you: for Allah is one Allah: 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 Allah as a Disposer of affairs.