There must have been a moment there, when Constantine the great converted the Roman empire into Christianity, that some of the Roman citizens and many around the ancient world were still pagan and seriously into worshiping multiple deities of many kinds.
At that moment some of the Romans needed to decide if they are joining in on the latest craze and going Christian or staying with the tradition of their ancestors by conserving their relationship with the other gods. The ones that broke from Paganism must have had a hard time, many daily rituals needed to stop (at least in public) and many old habits needed to disappear, however it seems that many of them moved forward and joined the monotheistic religion that marched all the way from Jerusalem.
1,700 years later many still praise the move from multiple deity religion to the one god religion, as if this was an improvement. The great Richard Dawkins has been known to say that Atheists just take one more step and let go of the idea of Gods altogether, and it makes perfect sense.
The blindness of those who are religious is much more evident when they talk about Paganism, it seems to them so crazy and ridiculous that anyone would think that there are many gods that they do not stop to think and consider why think there is even one god. Just take one step forward, it is clear that there are no multiple gods and it is clear that reducing them into one general god does not work either.