Polyphony is saved. :)

The mass was composed in honor of Pope Marcellus II, who reigned for three weeks in 1555. Recent scholarship suggests the most likely date of composition is 1562, when it was copied into a manuscript at the Basilica di Santa Maria Maggiore in Rome.[4]

The third and closing sessions of the Council of Trent were held in 1562–63, at which the use of polyphonic music in the Catholic Church was discussed. Concerns were raised over two problems: first, the use of music that was objectionable, such as secular songs provided with religious lyrics (contrafacta) or masses based on songs with lyrics about drinking or lovemaking; and second, whether imitation in polyphonic music obscured the words of the mass, interfering with the listener’s devotion. Some debate occurred over whether polyphony should be banned outright in worship, and some of the auxiliary publications by attendants of the Council caution against both of these problems. However, none of the official proclamations from the Council mentions polyphonic music, excepting one injunction against the use of music that is, in the words of the Council, “lascivious or impure”.[6]

Starting in the late 16th century, a legend began that the second of these points, the threat that polyphony might have been banned by the Council because of the unintelligibility of the words, was the impetus behind Palestrina’s composition of this mass. It was believed that the simple, declamatory style of Missa Papae Marcelli convinced Cardinal Carlo Borromeo, on hearing, that polyphony could be intelligible, and that music such as Palestrina’s was all too beautiful to ban from the Church. In 1607, the composer Agostino Agazzari wrote:

Music of the older kind is no longer in use, both because of the confusion and babel of the words, arising from the long and intricate imitations, and because it has no grace, for with all the voices singing, one hears neither period nor sense, these being interfered with and covered up by imitations…And on this account music would have come very near to being banished from the Holy Church by a sovereign pontiff [Pius IV], had not Giovanni Palestrina founded the remedy, showing that the fault and error lay, not with the music, but with the composers, and composing in confirmation of this the Mass entitled Missa Papae Marcelli.
— Quoted in Taruskin, Richard, and Weiss, Piero. Music in the Western World:A History in Documents. Schirmer, 1984, p. 141.

Jesuit musicians of the 17th century maintained this rumor, and it made its way into music history books into the 19th century, when historian Giuseppe Baini, in his 1828 biography of Palestrina, couched him as the “savior of polyphony” from a council wishing to wipe it out entirely:

On Saturday, 28 April 1565, by order of Cardinal Vitellozzi, all the singers of the papal chapel were gathered together at his residence. Cardinal Borromeo was already there, together with all the other six cardinals of the papal commission. Palestrina was there as well…they sang three Masses, of which the Pope Marcellus Mass was the last…The greatest and most incessant praise was given to the third, which was extraordinarily acclaimed and, by virtue of its entirely novel character, astonished even the performers themselves. Their Eminences heaped their congratulations on the composer, recommending to him to go on writing in that style and to communicate it to his pupils.
— Quoted in Taruskin, Richard, and Weiss, Piero. Music in the Western World:A History in Documents. Schirmer, 1984, p. 142.