Christianity absolutely has core doctrines that existed before the nicenene council- you're advancing a fiction. This idea of Christianity as a state religion is not historical and has no foundation.

Most of the major Pauline Epistles are mid first century, there were theological controversies as Christianity expanded but it's for sure not a state religion. It took centuries for the faith to dominate the pagan empire and the two got along pretty well well into late antiquity. Ultimately, many of the debates within Christianity in the fourth and fifth centuries are so obscure we can hardly understand them from a modern perspective- like the miaphysite controversy and countless others.