The King, Jesus, seated at the right hand of the Father, rises from His throne, removes His godly robe, and covers Himself with the sackcloth of human flesh, human nature, in the Incarnation, and sat down in the ashes of His Passion and crucifixion, although remaining sinless. He preaches repentance to us and dies for us, exclaiming that God will repent and turn from His fierce anger in order that we may not perish.
Taking our sins upon Himself, He prays: “Out of the depths I cry to thee, O Lord! Lord, hear my voice! Let thy ears be attentive to the voice of my supplications! If thou, O Lord, shouldst mark iniquities, Lord, who could stand? But there is forgiveness with thee, that thou mayest be feared. O Israel, hope in the Lord! For with the Lord there is steadfast love, and with him is plenteous redemption. And he will redeem Israel from all his iniquities.” (Ps 130: 1-4, 7-8)
Why would Jesus pray this psalm when He is holy, without sin? Because He became man, taking upon Himself sinful human nature. It is for this reason that He had to endure His Passion and death. He had to nail sinful human nature to the cross in order that He may exchange it for us with His divine nature. It is because of this that we may call God the Father our Father. We take Christ’s divine nature upon us in Baptism and the Eucharist. We are not clothed with His divine nature; therefore, if God the Father is our Father, then we are of Him and need to be like Him, being what we now are. This is what the Mass is about and what the Catholic Church is about.
What about our Blessed Mother? Did she pray this psalm? She was conceived immaculately, was without sin, had perpetual virginity; did she need to pray this psalm, and did she? Absolutely, yes. Everything she was and did was a gift from her Son, the Son of God. Therefore, she prayed, and prays, every psalm with us.
Glory be to the Father, and the Son, and the Holy Spirit. As it was in the beginning, is now, and ever shall be, world without end.