On (hopefully) at least a weekly basis Roman Catholics around the world say the Ave Maria (better known as the “Hail Mary”) in which they petition Mary, the Mother of God to intercede with him on our behalf.
It’s one of the best-known prayers in Catholicism, but for a period of Christian history the title theotokos or “mother of God,” was highly controversial. In the fifth century many Christians believed that to call Mary the mother of God was a nonsensical audacious claim bordering on heresy. In the theological struggle that ensued the fiercest proponent of the title was Cyril, bishop of Alexandria from 412-444. It is in large part thanks to Cyril that the phrase exists in Catholic doctrine and liturgy today.
Cyril was right to argue that Mary was the mother of god, philosophically speaking it's the only way for the incarnation to work (full disclosure: I'm a Mary loving Roman Catholic myself), but this theological genius, Doctor of the Church, and Roman Catholic and Orthodox saint had a very dark side.