I had a professor in a classical civ class ask how it was Christianity won out in ancient Rome. I've thought about that question for years and I think I have a good answer.
It became the religion of the ruling elite, and they eventually repressed the old ways.
Now adays we talk about "crypto currencies" and the like. The origin of the meaning of the word "crypto", as being something hidden, comes from the time when Pagans had to practice their religion in cemeteries, among the crypts. It gives you an idea of the repression.
They were called "CryptoPagans".