Why was the Pope Marcellus Mass performed without instruments?

The Pope Marcellus Mass was written in the a cappella style, which refers to the performance of choral music by unaccompanied voices.21

It is a common misconception that the mass was performed without instruments specifically because of a ban by the Council of Trent; rather, the a cappella style was a traditional "church style" of the era. While the Council of Trent did mandate that liturgical music be composed so that the sacred texts remained clearly understandable—avoiding excessive musical elaboration that might obscure the words—it did not explicitly forbid the use of instruments.2134

In fact, historical evidence suggests that in many Renaissance settings, including the Sistine Chapel, instruments such as the organ often "doubled" vocal parts to support the choir, even when no independent instrumental parts were written down. The Pope Marcellus Mass is celebrated for its masterful use of polyphony that balances artistic complexity with the clarity of text required by the church.12