Many, perhaps most, mainstream Christian churches define Christianity as Trinitarian, and that dogma is the “bright line” between Us & Them. It's absolutely not

Many, perhaps most, mainstream Christian churches define Christianity as Trinitarian, and that dogma is the “bright line” between Us & Them. It's absolutely not “recent”. You're confusing “additions/accretions to doctrine” with the way the Church worked in reality. Doctrines and dogmas were believed but not defined or formulated until heresies and controversies arose. The definition of a doctrine is a final way to settle a controversy in favor of orthodox belief. The definition of the Holy Trinity was simply when the reality was put into human words. Lest we forget that the Church produced Scripture, because it seems like folly for this to be turned upside down, but for 500 years, Scripture has begotten churches.