The Arian Controversy
The defining episode of Alexander's episcopate was his confrontation with Arius, a presbyter under his jurisdiction. Around 319 Arius began to teach a subordinationist account of Christ, holding that the Son was made rather than begotten and was not equal to the Father in substance. By tradition Alexander first attempted to recall him by gentler means, but Arius remained obstinate.
Alexander then summoned the clergy and bishops of his jurisdiction. The sources describe a synod at Alexandria, around 320, which condemned Arius, and a larger gathering of approximately one hundred bishops that placed him under anathema for denying that the Son was co-eternal and similar to the Father in substance. The controversy did not remain local: it divided churches across the East and drew imperial attention.