“From the very first, upon Our elevation to the chief Apostleship, We gladly turned our mind and energies and directed all our thoughts to those matters which concerned the preservation of a pure liturgy, and We strove with God’s help, by every means in our power, to accomplish this purpose. For, besides other decrees of the sacred Council of Trent, there were stipulations for Us to revise and re-edit the sacred books: the Catechism, the Missal and the Breviary. With the Catechism published for the instruction of the faithful, by God’s help, and the Breviary thoroughly revised for the worthy praise of God, in order that the Missal and Breviary may be in perfect harmony, as fitting and proper - for its most becoming that there be in the Church only one appropriate manner of reciting the Psalms and only one rite for the celebration of Mass - We deemed it necessary to give our immediate attention to what still remained to be done, viz, the re-editing of the Missal as soon as possible.Hence, We decided to entrust this work to learned men of our selection. They very carefully collated all their work with the ancient codices in Our Vatican Library and with reliable, preserved or emended codices from elsewhere. Besides this, these men consulted the works of ancient and approved authors concerning the same sacred rites; and thus they have restored the Missal itself to the original form and rite of the holy Fathers.”  Pope St. Pius V, Quo Primum, July 14, 1570