In the Abrahamic religions (Judaism, Christianity, Islam), having multiple faiths is explicitly forbidden in the Ten Commandments: https://en.m.wikisource.org/wiki/Bible_(King_James)/Exodus#20:1
2 I am the LORD thy God, which have brought thee out of the land of Egypt, out of the house of bondage.
3 Thou shalt have no other gods before me.
And you can see this play out historically: when Christians met peoples of other faiths, they didn't see it as an opportunity for learning and cultural exchange, they saw them as heathens that needed to be proselytized... or eradicated.
Conversely, I feel like Buddhists are the most likely to have other faiths, because Buddhism changed and adapted as it spread throughout Asia, and incorporated elements of the local religions.
The human body is 70% water. The energy required to evaporate all that would wipe out anything you'd get from combusting the organic matter. And it's not like they could leave them out back to dry out like seasoning firewood.