1. Church Replacement Theology — the idea of the Gentile church taking priority over the Jews — began by the mid-to-late 2nd century, brought about as the result of increasing anti-Semitism and Gentile pride, as the church became increasingly dominated by Gentile believers (who forgot what Paul said about pride, in Romans 11) and the apostate Jews became increasingly hostile and continued to persecute the Christians.
For additional reference, see Barry Horner’s Future Israel, chapter 2, “The Patristic Period,” p. 39
2. Amillennialism was invented by Augustine in the early 5th century. Augustine first believed the standard view of the time, premillennialism, but he was also influenced by Greek platonic thought — the same thinking that produced gnosticism, the basic idea that physical and material is evil, and spiritual, non-physical is good. Augustine was also morally repulsed by the behavior of some Christians, in a group called the Donatists, who took a rather carnal approach to the idea of the kingdom and enjoyed their love feasts (food, drink, revelry). This was also shortly after Constantine, and the church enjoyed the power, privilege and protection of the Roman Empire. Anti-Semitism was also quite strong, the Jews hated throughout the Roman world, and by that time they were scattered and clearly without power — so the biblical message that the despised and weak Jews would one day be given prominence and the other nations would come to them, appeared totally contrary (and unacceptable) to their observed world reality. So Augustine formulated the idea of the spiritual-only kingdom, relating it to the Church triumphant, the Roman church of his day. His new idea, amillennialism (which he described in “The City of God”), along with most everything else that Augustine taught, was incorporated into the Roman Catholic Church — and became (and still is) standard Catholic teaching.
Additional resources:
“The Allegorists Who Undermined the Normal Interpretation of Scripture,” by Mal Couch
Sourced From
Covenant Theology, Amillennialism and Replacement Theology: A Brief History