The transformation of Christianity from a persecuted sect to the Official Religion of the Empire changed the nature of theological debate. When the Emperor became the "Protector of the Faith," a theological disagreement became a State Crime.
The great Ecumenical Councils (Nicaea, Constantinople, Chalcedon) are often presented as gatherings of pious bishops seeking truth. Historically, they were Political Summits. The presence of the Emperor at these councils indicates that the primary goal was Pax Romana (Roman Peace). A divided church meant a divided empire.
The Mechanism of Exclusion: The process of declaring "heresy" followed a predictable pattern:
The "victory" of the Imperial Church came at a high cost: the loss of the Creative Tension that had characterized the first three centuries. By institutionalizing a single "correct" way to believe, the Church traded its diversity for stability. This created a pressure cooker of suppressed thought that would eventually explode in the Great Schism of 1054 and, later, the Protestant Reformation.