Proto-orthodox Christianity
Proto-orthodox Christianity or proto-orthodoxy was the early Christian movement that was the precursor of Christian orthodoxy. Older literature often referred to the group as "early Catholic" in the sense that their views were the closest to those of the more organized "Catholic" Church that was the State church of the Roman Empire during the 4th and 5th centuries.
Source: Wikipedia — Proto-orthodox Christianity (CC BY-SA 4.0)