32 Indeed, when he first came to Rome to receive from Pope St. Gregory II his mandate to preach the Word, Our predecessor, after he had examined, approved and praised Boniface, wrote to him with fatherly kindness: "Your zealous and Christ-directed designs, which have been declared to Us, and the praiseworthy demonstration of your upright faith demand that We use you as Our helper in spreading the word of God, which through His favor has been entrusted to Us. . . We rejoice in your faith, and We desire to cooperate with graces so generously given . . . Wherefore, in the name of the indivisible Trinity, and by the unshakeable authority of the Prince of the Apostles, Peter, with whose teachings and office We are entrusted by (divine) dispensation, and whose Holy See We govern, We invest your humble person with a religious mission, and instruct you to make known, with the persuasive powers of truth, by revelation of the name of Christ our Lord and God, the gospel of God's kingdom to whatever peoples, lost in the darkness of unbelief, you may by his Grace reach." Then, because of his outstanding virtues, having been consecrated bishop by Our predecessor, he pledged obedience to him and his successors, and solemnly declared: "I will keep in all its purity the Catholic faith and by God's grace persevere in the unity of that faith on which certainly depends the salvation of all Christians."
Source: Ecclesiae Fastos (Vatican.va)