In the very last scene the priest indicates to a layman, some rough and ready pagan type, that he is unsure that any good was done by seeking to convert the Indians. I don't remember the exact words. What I do remember is that they were a slap in the Face of the one Who said, "Make disciples of all nations."
By "existentialist" I mean a definite way of arguing from the particular (the failure of the missionary efforts depicted) to the general about the relativity of all Truth claims and the meaninglessness of attempts to propose as a Rock of Ages something that is like dust in the wind: no 17th century atheist would have spoken with the studied bleakness of this supposed 17th century priest. The movie was bogus historically and dramatically as well as theologically.
I don't remember anything good about this film other than the scenery and maybe the performances. I doubt that anything about the Blackrobes' effort is shown in a good light. I don't remember. Given the way in which it is all wrapped up, I doubt that they would have given the angel his due in any way. They did show the French, both clerical and lay, as having courage. But generally it was a put-down of Christianity which favored pagan naturalism.