Psalm 51.

Psalm 51: 1] Have mercy upon me, O God, according to thy lovingkindness: according unto the multitude of thy tender mercies blot out my transgressions. 2] Wash me throughly from mine iniquity, and cleanse me from my sin. 3] For I acknowledge my transgressions: and my sin is ever before me. 4] Against thee, thee only, have I sinned, and done this evil in thy sight: that thou mightest be justified when thou speakest, and be clear when thou judgest. 5] Behold, I was shapen in iniquity; and in sin did my mother conceive me. 6] Behold, thou desirest truth in the inward parts: and in the hidden part thou shalt make me to know wisdom. 7] Purge me with hyssop, and I shall be clean: wash me, and I shall be whiter than snow. 8] Make me to hear joy and gladness; that the bones which thou hast broken may rejoice. 9] Hide thy face from my sins, and blot out all mine iniquities. 10] Create in me a clean heart, O God; and renew a right spirit within me. 11] Cast me not away from thy presence; and take not thy holy spirit from me. 12] Restore unto me the joy of thy salvation; and uphold me with thy free spirit. (KJV)

Readers of this Blog will be most familiar with my love of this penitential psalm by that truly great Old Testament king and part time lowlife, David.
It encourages me immensely. If David could ask for - and expect - forgiveness from God for his murderous and adulterous behaviour - how uplifting it is for believers today to recognise that God still loves us and always offers us a way back.
Even the poetry of the words raises my spirits.

This psalm is just about as close to New Testament teaching as you can find in the Old.
( Note the link to Psalm 32.)