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How did the Council of Trent define the mass?

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Title page of a 1568 Welsh Christian doctrine book by Morys Clynnog, featuring Welsh text and a woodcut illustration of the crucifixion of Jesus at the center. — Counter-Reformation Title page of *Athravaeth Gristnogavl* (1568; “Christian Doctrine”), a Roman Catholic catechism translated into Welsh by Morys Clynnog as part of the church's Counter-Reformation efforts.

During the third period of the Council of Trent (1562–1563), the Roman Catholic Church formally defined the mass as a true sacrifice.1

This definition was a central component of the Church's response to the Protestant Reformation, which had challenged traditional Catholic teachings regarding the Eucharist and the nature of the liturgy. In addition to establishing this dogmatic definition, the Council sought to address the liturgical chaos of the era by issuing specific prescriptions concerning the form of the mass and liturgical music. These efforts ultimately led to the revision of the missal and the standardization of worship practices within the Church.21