I don't know that much about Descartes, but it's possible he did some of his writing in Latin. It would depend on when he did it and how he had been educated. A church background might very well have led to his writing in Latin.

I have seen it in French, but I was studying French at the time. Was it a translation or the original - je ne sais pas!

Was it Descartes whose famous last words were "I go now to the great perhaps"? If so, it would hardly indicate a religious background!