By all means, such a plague would wound the Empire, not not harm it, like you seem to imply.
Christianity at this point is much smaller than OTl, and is seen as radical sect of Judaism. Mithraism is also quite small, but has been growing popular in some areas such as the military.
A Mithraic Roman Empire instead of a Christian one would seem pretty fun.
Although if Christianity can go on big enough that Islam can get created but Rome is still Pagan then an Islamic part of Rome would be fun to see the civil wars and such that would come out of it.
Remember that the butterflies are so yuuge at this point that Islam will not exist.
Why? There is no reason to assume that a new religion would unify a disparate place like Arabia.True but someone in Arabia could do something similar to what Mohammed did
Why? There is no reason to assume that a new religion would unify a disparate place like Arabia.
By all means, such a plague would wound the Empire, not not harm it, like you seem to imply.