Check out the words to the music. Check out the dates of the composer’s birth and death. Notice that belief in the Assumption was not made up and imposed on the lay faithful by an overweening pope in 1954. It has been around at least since the 1500’s. And seeing that Palestrina wasn’t thrown out of the church on his ear due to the perceived heresy of this piece of music, chances are good that the belief was long-since established by the time he wrote this music.
I realize there are better and older evidences of the history behind this dogma. And any serious inquirer would do well to investigate them. The point I’m making is that we don’t even need those better and older proofs. The existence of this piece of music alone shows the claim that the pope made up the dogma of the Assumption for the lie that it is.