There are many compositions that use the text of "Ave Maria". One of them is "Ave Maria Guarani" from The Mission. Given how often classical compositions make it into movies, I've been wondering if this is not an arrangement of some existing version.
Is it Ennio Morricone's, as the rest of the soundtrack is? My research suggests that it may be, since it isn't Bach/Gounod, Schubert, Verdi, Cherubini, Brahms, Vavilov/Caccini, Skipper, Fauré, Biebl, or Arcadelt.
I'm fairly certain at this point, but Wikipedia isn't great at listing original composers, and given that the movie is set in the 18th century, there could be any number of lesser-known candidates that someone knowledgeable in classical music could have sourced in order to be true to the period.
Then again, given how many people have written an Ave Maria, maybe I shouldn't be surprised by another new one!