9 First We are convinced that the Church must look with penetrating eyes within itself, ponder the mystery of its own being, and draw enlightenment and inspiration from a deeper scrutiny of the doctrine of its own origin, nature, mission, and destiny. The doctrine is already known; it has been developed and popularized in the course of this century. But it can never claim to be sufficiently investigated and understood, for it contains "the publication of a mystery, kept hidden from the beginning of time in the all-creating mind of God . . . in order that it may be known . . . through the Church." (2) It is a storehouse of God's hidden counsels which the Church must bring to light. It is a doctrine which more than any other is arousing the expectation and attention of every faithful follower of Christ, and especially of men like us, Venerable Brethren, whom "the Holy Spirit has appointed to rule the very Church of God." (3)
Source: Ecclesiam Suam (Vatican.va)