You’re right. I don’t believe that. I don’t believe that we can know whose side G-d is on. We can only try to be on His side.
And, at the time, Catholicism was not morally superior to Islam, or Judaism, for that matter.
A lot of popular beliefs are wrong, such as the belief in the free lunch.
Even when the wind unexplainably shifts at a key point right before a naval battle?