Introduction to Arius
By the time Constantine took power the Church, which had always proclaimed belief in the divinity of Christ and the Trinity, had not as yet clearly defined how to officially and exactly explain the nature of its belief. Yes, of course, from the very beginning the Church believed and taught he was the divine Son of God. But, there were challenges. For example, the Gnostics twisted Christian doctrine into fitting into their philosophy, making Jesus an ethereal messenger, not really possessing human flesh, not really being human and dying, not really being divine. There were many forms of this doctrine.
Gnosticism was believable to many as it positioned itself as being part of Christianity, which it was not, because what the apostles taught was not clear in all places, and because what the Gnostics taught about God coincided with pagan beliefs about the gods that many peope already held. Indeed, numerous ancient people had a difficult time coming to terms with the Christian dogma of the Incarnation (as do many today). It was not unheard of that a god could become a man, but to do so, he or she would have to give up divinity in the process…like Hercules. But it was difficult to imagine that God could become a man and still remain God.
Yet, this what the Church has always believed and proclaimed. Jesus is True God and True Man. But explaining this mystery remained, well, a mystery. And so, well-intentioned but off-base teachers and thinkers emerged who began to assert notions to explain Jesus without really thinking through the implications of what they were saying. Such a person was a priest from Alexandria named Arius whose teaching about the Son of God caused a major crisis. At stake in this crisis was the monotheism of Christianity, the Christian concept of God as Trinity, and the identity of Christ and the Holy Spirit within this relationship.
In beginning to speak about Arius we need to grab some background information.

Bishop Meletius of Lycopolis
The first person in the drama leading to Arius is the Bishop of Lyocpolis named Meletius.
While the Donatist bishops were causing a schism in Carthage, Meletius, another rigorist bishop who formed a schismatic church during the persecution of Diocletian, was causing problems in other parts of Africa, particularly in Alexandria.
Meletius presumed to roam to dioceses that were not his own and ordain priests. This violated a long-standing Church tradition and law preventing bishops from doing exactly that. No bishop possessed the authority to ordain a priest in another bishop’s diocese without that bishop’s permission. That is still the case. One of the priests Meletius ordained in Alexandria without permission was a tall, thin Libyan named Arius who had already been excommunicated by Bishop Peter for his teaching about Jesus.

Meletius did not care because he was particularly critical of the Bishop of Alexandria, Peter, who not only had gone into hiding during Diocletian’s persecution, but who also was very lenient about allowing apostates back into the Church. It didn’t matter that Peter sent letters to Christians in prison giving them encouragement or that he undertook severe penances for the sake of the Church suffering during that time.
At any rate, after the Eastern Emperor Galerius and Maximinus resumed the persecution of Christians in 308 a.d. in the East, Bishop Peter did suffer martyrdom (in 311 a.d.) and at the same Church where St. Mark also suffered martyrdom, the Church at Baucalis. (Tradition holds that Bishop Peter was the very last Christian martyred during the persecutions of Diocletian and Galerius.) After Bishop Peter’s death many African bishops wrote letters demanding that Meletius stop ordaining.
Bishop Achillas of Alexandria

Succeeding Peter as the new Bishop of Alexandria was Achillas, who welcomed all the priests previously ordained by Meletius and “normalized” their ordinations. That normalization included the previously excommunicated Arius. And Arius received the pulpit of the prestigious Church of the Baucalis! There he enjoyed the presence of an audience where he preached his views on the nature of Christ, and he also began promoting himself as the next bishop. Achillas died in 313 a.d. and the man selected as his successor was not Arius but a priest named Alexander.
Soon, Bishop Alexander of Alexandria began hearing about a priest who had some strange notions about Jesus, and who, in promoting his teaching, was beginning to develop quite a following. As a result, Bishop Alexander invited Arius to a public discussion concerning his ideas to which Arius consented.