Why did the Romans stop believing in many gods and eventually believe in one god?

Answer 1

Until the fourth century CE, the vast majority of Romans continued to worship the old gods. Then, Constantine gave Christianity state patronage. It became politically and socially desirable for some, to adopt Christianity. The Church was also given the franchise to distribute state food aid to the poor, allowing it to proselytise to the poorer members of society.

Towards the end of the fourth century, after almost a century of largely Christian rule, the Roman Empire finally made Christianity the state religion. Public worship of the pagan gods was banned and overt persecution of pagans began.