The following comes from Fr. George Rutler:
The term ad limina refers to the tradition of diocesan bishops visiting the Pope every five years “at the threshold” of the Apostolic Palace to give an account of the state of their dioceses. This is not the same as going over a report card with the teacher, or an IRS audit. The Pope as Universal Shepherd guides and supports his fellow bishops in the care of the flocks committed to their charge. Because he is a successor of Peter, the Pope continues the commission given by our Lord: “But I have prayed for you, Simon, that your faith may not fail. And when you have turned back, strengthen your brothers” (Luke 22:32).
At the start of Lent, every Christian approaches the “threshold” of Christ and begins to walk toward Him for forty days, “o'er moor and fen, o'er crag and torrent,” examining the conscience. There is a paradox here, for as the faithful knock on His door, they find He has already been knocking on theirs. It is like the parable of the Prodigal Son who, once he turns and heads back to his father, sees his father approaching him.
Speaking from Heaven, the Risen Lord says, “Behold, I stand at the door and knock: if any man hear my voice, and open the door, I will come to him . . .” (Rev. 3:20). That is, He asks entry into the soul, seeking to open the intellect to the truth and to guide free actions along the way of that truth. In a reciprocal act, the acceptance of truth—and obedience to it—opens the soul to Him.
When I moved into my rectory, I found well over two hundred keys, all without labels. That was hardly useful. Likewise, Christ gave Peter the keys to Heaven, but, while Peter sublimely wielded those keys and proved worthy of the trust Christ placed in him, they are of little avail to us if we do not know what they are for.
There are keys that open ordinary doors and unlock earthly knowledge. Only the Church can open the eternal doors, and the blatant fact is that if human minds and wills open themselves to God, we will find that the gates of Heaven were open all the while. “Lift up your heads, O ye gates; and be ye lifted up, ye everlasting doors; and the King of glory shall come in” (Psalm 24:7).