There are two questions here (your initial post and follow up question). To your first question: Egypt was an agrarian/settled culture thanks to the Nile; they weren’t as interested in traveling/conquering/settling in new territories as the Greeks, and later the Romans, were, so their religious system wasn’t much dispersed beyond their settlements. There -were- some temples to Egyptian gods in Roman cities (see below for more on this). Greco-Roman religion was different from Egyptian religion in important ways: Egyptians worshipped gods that were half animal, half human, which the Greco-Romans considered uncivilized, maybe even backwards (hereafter I’m just going to say “Romans”). The Roman pantheon is filled with gods that are basically human, with superpowers. To Romans, animals were lower on the scale of being (Aristotle was a major source for understanding the hierarchy of beings, well past the classical era) so to have gods that were animal-hybrid mix was distasteful to them. That being said, Roman administration allowed fairly wide latitude in terms of worshipping the gods of choice—they had to be flexible about this from a practical standpoint as the Empire spread into foreign territories with indigenous, foreign gods. As the Empire weakened, it became more assiduous about ensuring that citizens, no matter what gods/goddesses they worshipped, also recognized the emperor as a supreme being (a few times yearly you might burn some incense for the Emperor—that was all that was required, but like the Pledge of Allegiance today, it indicated loyalty and respect.) All this is to say that yes, there were multiple religions operating within the Roman Empire, which were tolerated so long as fundamentally the administration was not threatened. Jews functioned within this system (the Christian New Testament describes this). Christianity was a small sect that was dealt with in various ways over its incipient phase(s). Pliny, a Roman civil servant in modern day Turkey, wrote a letter to Emperor Trajan asking what he should do about Christians in ca. 111-113 AD. Trajan (and after him, Hadrian) took lenient attitudes, but there were various leaders afterward who took a more severe stance—the major persecutions being the Decian (250 AD) and Diocletian (284 AD). The Diocletian persecutions were aimed at getting rid of minority religions across the board: so Christians, Manichees, Zoroastrians, et al., likely because the Empire was weakening (this kind of acceptance/purge would continue for a while though—don’t be deceived by convenient dates and descriptions—history is messy, just like now). The game changer was Constantine, who converted to Christianity and issued the Edict of Milan in 313 AD, thereby giving Christianity official state recognition. Because the Emperor backed it, the money flowed into Christian projects and establishments. Money was key. Some historians consider Christianity to have become the new face of the Roman imperialist project; based on the successful missionizing efforts in Northern Europe, it is easy to see why. However, there are a lot of issues and variables too complex to post here- suffice it to say, there are at least a million books written by scholars devoted to any one aspect of your multi-layered question. The “Fall of the Roman Empire” is an easily digested trope, but it didn’t happen all at once, or for any one reason. The process was gradual and complex, and the adoption of Christianity is part of that complex process. Peter Brown (Princeton) writes about exactly this transitional period in a way that is interesting, lively, and comprehensive. Check out pretty much any of his books to get more information about all of this.