37 The mystery of the Church is not a truth to be confined to the realms of speculative theology. It must be lived, so that the faithful may have a kind of intuitive experience of it, even before they come to understand it clearly. And the faithful as a community will indeed recognize that they belong to Christ's Mystical Body when they realize that a part of the ministry of the Church's hierarchy is to initiate men into the Christian way of life, to beget them, (26) teach them, sanctify them, and be their leaders. The hierarchy is a sort of instrument fashioned by Christ, which He Himself uses to communicate to His mystical members the marvelous gifts of truth and grace. He uses it, too, to impart an external, visible structure to the Mystical Body in its pilgrimage through the world, and to give it its sublime unity, its ability to perform its various tasks, its concerted multiplicity of form, and its spiritual beauty. Images are powerless to convey to the mind an adequate notion of the reality and sublimity of this mystery, but having mentioned the image which St. Paul used, that of the Mystical Body, We should also make mention of the image used by Christ, that of a building, of which He is Himself the architect and builder. Though He founded this building on a man who was naturally weak and frail, Christ transformed him into solid rock, never to be without God's marvelous support: "Upon this rock I will build my Church." (77)
Source: Ecclesiam Suam (Vatican.va)