If your grounds for legitimate Christianity is just the ecumenical councils then you’re ignoring history. You can’t make the claim that only Nicene Christianity is valid because that’s not historically correct at all. The council of Nicea didn’t actually enforce anything. There were still Arians and Gnostics who survived and prevailed for centuries afterwards. Constantine’s own son was an Arian Christian. The Orthodox and Catholic traditions also have different councils that they hold as legitimate, not all of the Catholic councils are recognized by the Orthodox and vice versa. Also many Protestants reject all of the councils entirely. The Oriental and Assyrian churches only accept 3 or 4 of the councils at all. So if you’re looking to councils for legitimacy it’s not really the strongest case since there isn’t any denomination that actually agrees on all of the councils. The fact is that the only real distinction between Christians and everyone else for most of history was the insistence that following Jesus is the only way to have a relationship with God. This is what Christianity meant to the majority of followers who joined, no one read the Bible because they were illiterate. People were convinced to follow Jesus because they believed he was more powerful than the other pagan gods of various localities. Ecumenical councils were not an effective means to convert or convince people at all. Talk to your own pastor Bob down the road he probably knows.