Arianism
In 312 AD, Christianity finally became a recognized, legitimate religion after its acceptance by Emperor Constantine. Not long afterward, a dispute blew up over the exact nature of Christ that was to determine the critical canonical beliefs of the Catholic Church. On the one side were the Homoousians who believed in the Holy Trinity of Father, Son, and Holy Spirit. On the other was Arianism. Unlike adoptionism, Arianism completely denied Christ any divinity, reducing him to the status of a mere creation. Instead, their focus was on god alone.
A Libyan theologian named Arius who lived between 250and 336AD founded Arianism. In 319, when Arius was a priest in Alexandria, Egypt, he became embroiled in an argument with his bishop about the divinity of Christ. Arius argued that Christ was not God’s equal because he was not of the same substance or homoousion as God. For that reason, Christ was not immortal or divine. Instead, he was merely the highest of God’s creations. Arius’s ideas were attractive to many, and the priest quickly discovered he had amassed quite a following in Egypt, Syria, and Asia Minor.
However, Arius’s ideas were anathema to the established church. By denying Christ’s divinity, the Arians were reducing him to a demigod. As Christ was still worshipped, declaring him separate from God was to effectively reinstate polytheism, leaving the church one step away from paganism. The philosophy also undermined the whole concept of Christian redemption, as only God could reconcile humanity with God- not a mere mortal, however perfect. So, in 321AD, the Bishops of Alexandria convened a Synod to deal with the Arian movement- and excommunicated Arius.
Arius however, had the support of Bishop Eusebius of Nicomedia. Eusebius gave Arius sanctuary and sponsored another Synod, which convened in Bithynia in 323AD that reinstated Arius to the church. Two years later, the pair faced a much more robust challenge to their beliefs. In an attempt to settle the matter between the Arians and Homoousians, Emperor Constantine called a Council at Nicaea. 318 mainly eastern Bishops attended, but despite Arianism’s massive support in the Eastern Roman Empire, the Council found in the Homoousions favor. The council declared Arianism a heresy and Arius and the two bishops who supported him were banished.
Eusebius fought on. He managed to arrange Arius’s recall from exile and reinstatement into the church in 334AD. When the Bishop of Alexandria, Athanasius refused Arius the sacraments, Eusebius persuaded the emperor to intervene. Athanasius backed down- only for Arius to die two days before he was due to take communion. After Arius’s death, Eusebius kept his beliefs alive for a time, but in 381AD, the Church finally suppressed Arianism at the first Council of Constantinople. The Homoousians had finally won. But Arianism as a heresy continued until 662Ad when the German Lombard’s became the last Arian sect to submit to church authority.
Meanwhile, other ‘dangerous’ heresies continued to appear- some even going so far as to deny humanity’s inherently sinful nature- and dare to suggest people had free will.