Well Christianity became dominant religion around 350 ad, n Rome was pretty much done around 500 ad
You're watching it wrong, romans were changing due years of peace and perceived safety (in comparision of Rome's origin), and were switching away anyway from the ancient Jupiter or the harsh Mars
Perceived safety is important because the peace created by Rome power changed the perspective from
-Be a legionary, protect Rome and romans from enemies
to
-Be a legionary, Rome is'nt in danger and your peoples neither. But go die for Rome's expansion, it's cool
The more romans detached from Rome, the more they detached from religions strongly related to roman's identity
Christianity attracted them but you got romans (specially soldiers) attracted to Mithraism for example, romans were changing and found new religions that appealed them, they was'nt changed from the religions to begin with
What changed was that loss of roman pride/identity and loss of survival istinct had peoples just losing patriotic duty sense an just chasing individualistic higher standard of living.
When Rome split in two (huge weakening of the empire), nobody of middle/low levels had fucks to offer
When most of late "roman" armies were made of barbarians who later revealed to be unloyal (youdontsay) everybody were like "cool let them do it, so i will not have to do live hard legionary style and chase my dream of be a youtuber"
The roman was changed living the false perception that a Rome was something indestructible that did'nt actually needed roman's protection, and that it's fall was something impossible
Romans lost strong roman values (born into harsh times) in favor of softer, open, individualistic mindset (born into "peace" times)
The irony is that when shit started to look bad they did'nt tried to return to the roman values that made them strong, but sticked to new hipster shit and watched theyr world slowly fall into shit
Go tell to first romans to entrust germanic fucks for the "protection of Rome" lol
"Peoples" stopped value theyr nation existence as something to defend
West did'nt changed that much since then
Blame teh Jesus or barbarian invasions (lol, it's not like there were actual romans to fight them) or whatever, is just lose vision of the main cancer and focus on collateral factors