Sure thing. There’s actually a great book on this entitled The Rise of Christianity by Dr. Rodney Stark. In the book, Stark does the math, and shows that a social movement numbering only 1,000 people in 40 A.D. could easily grow to 25 or even 35 million by the fourth century, despite all of the challenges of the ancient world — if the members of the movement lived according to the principles he laid out which are:
- Social networks
- Caring for the sick, widows, and orphans
- Stance against adultery, abortion, and infanticide (the ancient world was not kind to women and children)
- A theology of love
Putting it all together he shows a very tangible demographic results: 40% growth per decade for hundreds of years. By the time Constantine got on board, the movement was already well under way. The Edict of Milan just made Christianity tolerable in the ancient world instead of blatant persecution. After it became state sponsored, you are quite correct that that is when things went to hell and have since plagued us ever since!