“By schisms rent asunder, by heresies distressed.”

Those lyrics, from the third verse of Samuel John Stone’s “Church’s One Foundation,” written in 1866 to rebut heresies about biblical inspiration then ripping through the Church of England, have a timeless quality about them.
They could have been written in A.D. 48–50 when the nascent church convened in Jerusalem to disabuse adherents of the notion that new Gentile believers had to adopt Jewish ways (see Acts 15). And they could have been repeated in every century since, starting already in the next one.
A second-century teacher named Marcion tried to scrub Scripture of the Old Testament — he thought it was about a different God from the New Testament God, an evil creator god of the Jews. Gnosticism, also from the early centuries of the church, held that special spiritual knowledge (gnosis), apart from that revealed in Scripture, was necessary for salvation. Then there was Montanism and Donatism and Docetism and Apollinarianism... There is no end of heretical “isms.”
But the major heresy of the church’s first four centuries was Arianism. Arius, a presbyter from Alexandria, held that Jesus was a created being, not co-eternal with God, and thus not fully God. Said he: “If the Father begat the Son, then he who was begotten had a beginning in existence, and from this it follows there was a time when the Son was not.”
Arianism was no small offshoot of the orthodox faith. Arius was no ascetic crank hurling anathemas from atop a desert pillar. Arianism claimed thousands of followers from all stations of life, from slaves to sovereigns. Indeed, Emperors Constantius II and Valens subscribed to the teachings of the subordinationists, and priests, monks, bishops, and whole congregations aligned with the anti-Trinitarians.
So riven was the faith that a church-wide council was called by Emperor Constantine. That convention, labeled the Council of Nicaea, brought about 300 church bishops together in A.D. 325 in the Bithynian town...