I'm saying that Christianity at that point had already spread its tendrils as far south as Ethiopia, as far east as India, as far west as Scotland, and as far north as Germany. It was not the adoption of Nicene Christianity by Rome which decided the dominance of non-native religions in the region's history; only the dominance of Nicene Christianity specifically in Christianity of the region.

Traditional European religions lacked the philosophical and cultural defenses necessary against the proselyting of Christianity, whose appeal, furthermore, intensified in times of worldly crisis. The Empire itself adopting Christianity sped up the process - and empowered Christians to act with impunity in the last ~150 years of the dying Empire's life - but was not a fundamental turning point, especially as Christianity at that point had already established church structures which gave it the power of a state within a state.