Fourth century heresy, according to the Roman Catholic Church, named for its creator, Arius. Arianism denies the co-equal divinity and co-eternal existence of Jesus Christ, relegating him to a sort of second in command, existing before Creation but after God. (Here's his logic: God is unbegotten, Jesus is begotten, therefore Jesus cannot be equal to his father.)

Though condemned by the Council of Nicaea in 325, Emperor Constantius II and bishop Eusebius of Nicomedia were Arians, and by 359, it was official doctrine of the Eastern Roman Empire. Infighting between the semi-Arians and the neo-Arians after Constantius died weakened its hold. Theodosius I outlawed this doctrine in 379 and made the Nicene Creed orthodoxy, but communities of Goths and Vandals converted by Arian bishops kept it alive for two more centuries.