8 In short, Venerable Brethren, there are three policies which principally exercise Our mind when We reflect on the enormous responsibility for the Church of Christ which, unsought and undeserved, the providence of God has laid upon Us in making Us Bishop of Rome, successor to St. Peter the Apostle and Key-bearer of the Kingdom of Heaven, and Vicar of Christ who appointed Peter the first Shepherd of his worldwide flock.
Source: Ecclesiam Suam (Vatican.va)