The Libyan theologian Arius (died ca. 336) was presbyter of the Christian Church in Alexandria and the first of the great heresiarchs.

Nothing is known of the early life of Arius except that he may have been born in Libya and may have studied under Lucian, the revered teacher and martyr of Antioch. It is certain that he was pastor of the Baucalis church on the Alexandrian waterfront, where he won many supporters by his preaching. He may have aspired to the episcopacy in Alexandria, which went instead to his fellow presbyter, Alexander.

Not until 318, however, did Arius become prominent and then only as a heretic. He began by criticizing the Trinitarian views of Bishop Alexander, accusing him of Sabellianism (an early heresy which did not distinguish clearly between the "Persons" of the Trinity). But when Arius explained his position, he caused greater alarm with his own views, and...