Be gentle, guys, it's my first time, but I think this thread is riddled with errors.

Basically, the OP is asking reddit to ELI5 why the Christianity replaced the various Roman religions as the most popular religion, why the Romans "dropped" their gods to follow the new faith. Some commenters are insisting it's because Rome really wasn't practicing religion seriously anymore, so it was easy to drop the old ways. That's not consistent with anything I've read, describing a time and place with a great deal of religious pluralism but serious religious commitment - hell, you had to be religious to be in the Senate and serve as consul. But not according to one commenter:

...we can safely assume that at the time of Constantine few if any Romans believed in the literal existance of the twelve olympic gods. The predominant belief system of the Roman empire at the time was probably a mix of philosophical scepticism and newly imported middle-eastern cults such as Mithraism, Zoroastrianism and Christianity.

This doesn't seem to agree:

https://en.wikipedia.org/wiki/Religion_in_ancient_Rome

For ordinary Romans, religion was a part of daily life.[1] Each home had a household shrine at which prayers and libations to the family's domestic deities were offered. Neighborhood shrines and sacred places such as springs and groves dotted the city.[2] The Roman calendar was structured around religious observances. Women, slaves, and children all participated in a range of religious activities. Some public rituals could be conducted only by women, and women formed what is perhaps Rome's most famous priesthood, the state-supported Vestals, who tended Rome's sacred hearth for centuries, until disbanded under Christian domination.

I'm not sure what this next guy is driving at, but he seems to be suggesting politics replaced religion, that state authorities - because they were on the money - were the new gods:

Simply put, it wasn't working; neither for the citizens or the ruling classes. It represented a paradigm shift in existential thought moving away from agricultural/nature worship (animal sacrifices, nature figureheads, etc) to socio-political symbolism and authority. This was a necessity when dealing with large, multi-cultural populations assimilated into urban coin based economies brought about by significant Roman conquest.

Others are saying Christianity was "basically the same" as the religion(s) it replaced. In my opinion, Gibbon provided the best answer to that and the whole thing:

https://en.wikipedia.org/wiki/Early_Christianity#Spread_of_Christianity

Edward Gibbon (1737-1794), in his classic The History of the Decline and Fall of the Roman Empire (1776-1789), discusses the topic in considerable detail in his famous Chapter Fifteen, summarizing the historical causes of the early success of Christianity as follows: "(1) The inflexible, and, if we may use the expression, the intolerant zeal of the Christians, derived, it is true, from the Jewish religion, but purified from the narrow and unsocial spirit which, instead of inviting, had deterred the Gentiles from embracing the law of Moses. (2) The doctrine of a future life, improved by every additional circumstance which could give weight and efficacy to that important truth. (3) The miraculous powers ascribed to the primitive church. (4) The pure and austere morals of the Christians. (5) The union and discipline of the Christian republic, which gradually formed an independent and increasing state in the heart of the Roman empire."[67]