Sure the Normans were nominally Christian / Catholic themselves, some were even pious.
But Pope Leo thought the power of God would allow him and an army of Italians to prevail militarily against the Normans. His army was convinced it was true as well, right up until the Norman charge and rout. So much for that theory. ;)
I tend to doubt that the Pope believed that, whatever he said to the troops to encourage them. In any case the belief is that 'the Gates of Hell' will not prevail against the Church, not that the temporal power of the Pope would prevail against that of other members of the Church.