The priest Arius caused what became known as the Arian controversy in the 4th Century AD, and he is considered to be a heretic by both the Roman Catholics and the Orthodox Church.
This is because Arius believed that there was a time when Jesus did not exist co-eternally with the Father, and that He was not of the same nature, being and essence as the Father. The Ecumenical Council of Nicea and in particular, St Athanasius, rejected these beliefs as being totally false.