L'homme armé
"Missa Papae Marcelli" does not (as far as is known) make use of any pre-existing theme. The motif of a rising perfect fourth and stepwise return (illustrated) is used extensively throughout this mass. It is similar in profile to the opening of the French secular song "L'homme armé", which provided the theme for many Renaissance masses. But this is probably a coincidence, as themes with this profile were common in the 16th century, and Palestrina himself used them in several other masses.