109 Mary, the mother of the Lord stood by her Son's Cross; no one has taught me this but the holy Evangelist St. John. Others have related how the earth was shaken at the Lord's passion, the sky was covered with darkness, the sun withdrew itself; that the thief was after a faithful confession received into paradise. John tells us what the others have not told, how the Lord fixed on the Cross called to His mother, esteeming it of more worth that, victorious over His sufferings, He rendered her the offices of piety, than that He gave her a heavenly kingdom.
For if it be according to religion to grant pardon to the thief, it is a mark of much greater piety that a mother is honoured with such affection by her Son. “Behold,” He says, “your Son”....“Behold your mother.” Christ testified from the Cross, and divided the offices of piety between the mother and the disciple. The Lord made not only a public but also a private testament, and John signed this testament of His, a witness worthy of so great a Testator. A good testament not of money but of eternal life, which was written not with ink but with the Spirit of the living God, Who says: “My tongue is the pen of a quickly writing scribe.”
Source: Letters (New Advent)