Have mercy on me, O God, according to Thy great mercy; and according to the multitude of Thy compassions blot out my transgression. Wash me thoroughly from mine iniquity, and cleanse me from my sin. … Turn Thy face away from my sins, and blot out all mine iniquities. Create in me a clean heart, O God, and renew a right spirit within me. Cast me not away from Thy presence, and take not Thy Holy Spirit from me. Restore unto me the joy of Thy salvation, and with Thy governing Spirit establish me.  … For if Thou hadst desired sacrifice, I had given it; with whole-burnt offerings Thou shalt not be pleased. A sacrifice unto God is a broken spirit; a heart that is broken and humbled God will not despise.

- From Psalm 50 (51 MT)

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David was living at that time when sacrifices of victim animals were offered to God, and he saw these times that were to be. Do we not perceive ourselves in these words? Those sacrifices were figurative, foretelling the One Saving Sacrifice. Not even we have been left without a Sacrifice to offer to God. For hear what he says, having a concern for his sin, and wishing the evil thing which he has done to be forgiven him: 

“For if Thou hadst desired sacrifice, I had given it; with whole-burnt offerings Thou shalt not be pleased.”

Nothing shall we therefore offer? So shall we come to God? And whence shall we propitiate Him? Offer; certainly in yourself you have what you may offer. Do not from without fetch frankincense, but say, “In me are, O God, Your vows, which I will render of praise to You.” Do not from without seek cattle to slay, you have in yourself what you may kill. 

 - St. Augustine of Hippo

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… that you put off, concerning your former conduct, the old man which grows corrupt according to the deceitful lusts, and be renewed in the spirit of your mind, and that you put on the new man which was created according to God, in true righteousness and holiness. 

- St. Paul, Epistle to the Ephesians 4:22-24