Its impossible to believe... knowing how the ancient Catholics pursued 'Christian' heretics... virtually destroying any non conforming sect of Christianity... that Pagans weren't treated in exactly the same manner. At least in some, if not many cases.
We know that Constantine, and his bishops, as well the Christian emperors that followed, destroyed Pagan temples, banned rituals, and essentially forced urban pagans to become Christian. The extent of the Pagan persecution that occurred is virtually unknown, thanks largely to the fact that the victor wrote the history and destroyed untold remnants of divergent evidence. Considering the behavior of men throughout history, including the often vicious church, one might say its naive to think that treatment wasn't brutal at times.
Sure the Muslim Arabs violently conquered the mideast, North Africa and parts of Spain.
The Christians returned the favor with several centuries of Crusades and the Spanish Inquisition.
Some conversions were assuredly peaceful, others were not.
The Native tribes of the Americas, especially South and Latin America might argue that their 'conversion' came with a hefty price.