Earle Cairns discusses many places where historians believed the division between ancient and medieval church history occurred. He settles on the time when Gregory I became Bishop of Rome.
At this point, the Catholic church grew more Roman, but Gregory laid down the main theological idea that reigned until Thomas Aquinas started writing.
I intend to read more about Gregory as he is a key figure. He, Jerome, Ambrose, and Augustine are considered the four doctors of the Western church. He is a man I greatly respect.
He began major doctrines I do not agree with that still exist. Like his peers, he took too much time finding allegories in Scripture that were not necessarily intended. Although he liked Augustine, he softened his predestinarian view by saying that man is not guilty because of Adam but only sinful. He took the Church in a semi-Pelagian direction. He is the one that began to base God's grace not on God, but on man's merit. He honed the Purgatory doctrine and reinterpreted the Mass as an ongoing sacrifice of Jesus. He also did not take time to learn the Bible in its original languages. Having not done that myself, it makes me wonder if I should.
Things I like about Gregory: despite his skewed soteriology, Gregory did believe in salvation through Christ's blood. He also converted many regions of the world to Christianity, including England. I loved the story of how he noticed the Anglo-Saxon slaves being sold and decided that they were not "Angles" but "Angels". That is how he started to evangelize he Celtic world. He also fought to stop Arian influences and disapproved of the bishop of Constantinople claiming a universal bishopric for himself. He did not believe in such a thing. Hopefully, as history continues and winds down, the Church can remember this passion for evangelism and regain unity under a historical-grammatical interpretation of Scripture.