[VOA]

...there was a time when atheists fed Christians to the lions...

By and large, the Romans feeding Christians to lions (and other creatures)
were polytheistic.

I've heard some church historians say that the average Roman might
accept that the Christians were leading exemplary lives...but why
couldn't these weirdos accept that there were a multiplicity of
gods and Caesar was also a god?

To those Romans, the Christians just didn't have enough religion (or gods).

[Virginia-American]

To those Romans, the Christians just didn't have enough religion (or gods).

In fact, ironically enough, the charge against early Christians was often atheism.