depends on your perspective. one could argue 325 ad (first council of nicaea), possibly when rome adopted christianity as the official religion (380 ad) or maybe even sometime later. we don't have a good firm date. what is known is that the system of having a single bishop did not come into place until the 2nd century sometime. the early church was not anything like the modern catholic church's structure. there were many different local presbyters and churches that operated independently in the time. the modern catholic church was essentially formed after a long run of a consolidation of power made possible by constantine.
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