39 Of them, more than of the rest, it can be said that Christ who is the Head of the Mystical Body "requires them to be His members . . ., first of all, because in so far as the Person of Christ is represented by the Sovereign Pontiff, the latter must call upon not a few others to share his anxieties lest he be overwhelmed by the burdens of his pastoral office."
Source: Fidei Donum (Vatican.va)