Quo Primum is the bull by St Pius V that promulgated the Tridentine liturgy and made it the official rite of the Catholic Church, with some exceptions.
Quo Primum is a very important document for today, considering that Pope Francis released the motu proprio Traditionis custodes, which restricted the Traditional Latin Mass.
Quo Primum (From the First)
His Holiness Pope St. Pius V
Bull Promulgated on July 14, 1570
Upon Our elevation to the Apostolic throne, We gladly turned Our mind and energies, and directed all Our thoughts, to the matter of preserving incorrupt the public worship of the Church; and We have striven, with God’s help, by every means in Our power to achieve that purpose.
Whereas amongst other decrees of the holy Council of Trent, We were charged with revision and re-issue of the sacred books, to wit, the Catechism, the Missal and the Breviary; and whereas We have with God’s consent published a Catechism for the instruction of the faithful and thoroughly revised the Breviary for the due performance of the Divine Office, We next, in order that the Missal and Breviary might be in perfect harmony, as is right and proper (considering that it is altogether fitting that there should be in the Church only one appropriate manner of Psalmody and one sole rite of celebrating Mass), deemed it necessary to give Our immediate attention to what still remained to be done, namely the re-editing of the Missal with the least possible delay.
We resolved accordingly to delegate this task to a select committee of scholars; and they, having at every stage of their work and with the utmost care collated the ancient codices in Our Vatican Library and reliable (original or amended) codices from elsewhere, and having also consulted the writing of ancient and approved authors who have bequeathed to us records relating to the said sacred rites, thus restored the Missal itself to the pristine form and rite of the holy Fathers. When this production had been subjected to close scrutiny and further amended We, after mature consideration, ordered that the final result be forthwith printed and published in Rome, so that all may enjoy the fruit of this labor; that priests may know what prayers to use, and what rites and ceremonies they are to observe henceforward in the celebration of Masses.
Now therefore, in order that all everywhere may adopt and observe what has been delivered to them by the Holy Roman Church, Mother and Mistress of the other churches, it shall be unlawful henceforth and forever throughout the Christian world to sing or to read Masses according to any formula other than that of this Missal published by Us; this ordinance to apply to all churches and chapels, with or without care of souls, patriarchal, collegiate, and parochial, be they secular or belonging to any religious Order, whether of men (including the military Orders) or of women, in which conventual Masses are or ought to be sung aloud in choir or read privately according to the rites and customs of the Roman Church; to apply, moreover, even if the said churches have been in any way exempted, whether by indult of the Apostolic See, by custom, by privilege, or even by oath or Apostolic confirmation, or have their rights and faculties guaranteed to them in any other way whatsoever, saving only those in which the practice of saying Mass differently was granted over 200 years ago simultaneously with the Apostolic See’s institution and confirmation of the church, and those in which there has prevailed a similar custom followed continuously for a period of not less than 200 years; in which cases We in no wise rescind their prerogatives or customs aforesaid. Nevertheless, if this Missal which We have seen fit to publish be more agreeable to these last, We hereby permit them to celebrate Mass according to its rite, subject to the consent of their bishop or prelate, and of their whole Chapter, all else to the contrary notwithstanding. All other churches aforesaid are hereby denied the use of other missals, which are to be wholly and entirely rejected; and by this present Constitution, which shall have the force of law in perpetuity, We order and enjoin under pain of Our displeasure that nothing be added to Our newly published Missal, nothing omitted therefrom, and nothing whatsoever altered therein.