The concept of "heretics" was indeed too broad.
Initially, it referred to the defamers of the Church, but after the latter were slaughtered to the brink of extinction, anyone identified by the Church's Round Table was prosecuted as heretics. During the height of the trial movements, anyone with dissenting views was subject to trial as a heretic. For a considerable period, everyone in the Church was in constant fear for their safety, carefully watching their words and actions, afraid to give others anything to use against them.
Before the battle with the Mechanical God's religion began, the Church's higher-ups had already commenced their struggle for "preaching territories," with each meeting ending in discord.
Nobody ever took the Mechanical God's religion seriously.
Undoubtedly, internal strife was one of the main reasons that led to the Church's downfall. As far as he currently knew, the destruction of the upper world was also very likely a result of internal conflict.