Quo Primum (from the first) is the name of an Apostolic constitution in the form of a Papal bull issued by Pope St. Pius V on 14 July 1570. It promulgated the 1570 edition of the Roman Missal, and made its use obligatory throughout the Latin Rite of the Catholic Church, except where there existed a different Mass liturgy of at least two hundred years' standing.
The declared reason for this measure was: "It is most becoming that there be in the Church ... only one rite for the celebration of Mass." However, he did make the exception mentioned, which permitted the survival, within limited areas or in celebrations by members of certain religious orders, of Latin liturgical rites other than the Roman Rite, rites such as the Ambrosian and Mozarabic Rites, that of the Diocese of Lyon and certain Catholic Order Rites. Some of these dioceses and religious orders have since decided to adopt the Roman Rite. Others preferred not to avail themselves of the exemption to which they were entitled and instead to adopt the Roman Missal immediately.
Thus, although the bull Quo primum contained expressions such as "Let all everywhere adopt and observe what has been handed down by the Holy Roman Church, the Mother and Teacher of the other Churches, and let Masses not be sung or read according to any other formula than that of this Missal published by Us. This ordinance applies henceforth, now, and forever, throughout all the provinces of the Christian world", exceptions were allowed from the start.
In the bull Pope Pius V declared: "By this present Constitution, which will be valid henceforth, now, and forever, We order and enjoin that nothing must be added to Our recently published Missal, nothing omitted from it, nor anything whatsoever be changed within it." And he concluded: "No one whosoever is permitted to alter this notice of Our permission, statute, ordinance, command, precept, grant, indult, declaration, will, decree, and prohibition. Should anyone dare to contravene it, let him know that he will incur the wrath of Almighty God and of the Blessed Apostles Peter and Paul."
By this, he forbade alterations by other authorities, ecclesiastical or civil, or by private individuals, but obviously not by himself or by his successors, whose authority as Popes were equal to his own. He himself altered his Missal when, after the victory of Lepanto in the following year, he added to it the feast of Our Lady of Victory. In 1585, Pope Sixtus V restored the feast of the Presentation of the Virgin Mary, which Pope Pius V had removed from the Missal. Only 34 years after the publication of Quo primum, Pope Clement VIII made a general revision of the Roman Missal, as did Pope Urban VIII 30 years later.
More specifically, the purpose of Quo Primum was to reinforce the canons and decrees of the Council of Trent which removed from the Mass, prayers that had been added over the years, although well-meaning, nevertheless attached local intentions. Although having been known to be somewhat of a concilliarist prior to ascending to the papacy, once there, he recognized the need to unify the flock, having been well aware of Luther's Reformation.