During the reign of Diocletian, the greatest persecution of Christians occurred in the Roman Empire. He required all subjects to sacrifice to the Roman Gods ( of course true Christians would refuse and many bishops succumbed to the pressures ) and blamed Christians when bad things occurred. He sent a universal edict that Christianity was to be eliminated. Before he retired from being sole emperor of Rome, he divided the Roman Empire into 4 parts and put a general in each part to rule each part. After Diocletian retired, these generals fought each other and when one of them, Constantius, died, his son Constantine, took over and eventually became the sole ruler of the Roman Empire from 306 AD to 337 AD. Constantine's mother, Helena was a Christian and in 313 AD Constantine issued the Edict of Milan which legalized Christianity. By 325 AD Christianity was a major religion, but there were many factions competing with each other. Constantine was a sun worshiper. His coins had images of him with the sun god next to him. He wanted to keep his empire united so he sent 1800 invitations to Christian bishops from all over the empire to come to Nicaea to discuss topics in the Bible and come to a consensus. Sebellianism (the belief that the one God takes on the role of Father, Son, and Holy Spirit) was already declared a heresy. The topics discussed during the council was Arianism, whether or not to accept bishops who recanted their faith under severe persecution back into the fold, and when to celebrate Easter. Arius believed that Christ was a created being made from a different substance from the Father. Around 300 bishops attended the Council of Nicaea and Constantine oversaw the proceedings. They declared Arianism a heresy so Constantine banished him, had his writings burned, and forbade possession of his writings. Meanwhile, the bishops at the council had to come up with a consensus on the divinity of Jesus. They believed that Jesus was God, the Father was God, and the Holy Spirit was God, but that went against the Jewish belief in one God. Also, they didn't want to fall into the category of Sebellianism, so they came up with the Trinity which states that there is only one God that is made up of three distinct persons. Each is the one God. Sounds like Sebllianism to me.
The Nicene Creed was adopted and this consensus was decreed to be taught by all bishops. The Roman Catholic Church adopted the consensus and declared Constantine a saint. They call him Saint Constantine although he was never canonized a saint. He had his son killed for what his wife told him, and then, when he found out that his wife had lied, he had her tossed into a vat of boiling water. It was in his later years that Eusebius took notes from him to write his biography. It is then that he relates his dream about conquering under the Christian banner. It is in his deathbed that he gets water baptized. He wanted to wait till then so that all the evil things that he did during his reign would be forgiven. Arianism persisted after the Council of Nicaea and eventually one of the later emperors was an Arian. Also, Arius was allowed back after making some modifications which satisfied the emperor Constantine.