An ascetic form of penance is to act penitent or remorseful and punish oneself (see Colossians 2:18-23 Colossians 2:18-23  Let no man beguile you of your reward in a voluntary humility and worshipping of angels, intruding into those things which he has not seen, vainly puffed up by his fleshly mind,
 And not holding the Head, from which all the body by joints and bands having nourishment ministered, and knit together, increases with the increase of God.
 Why if you be dead with Christ from the rudiments of the world, why, as though living in the world, are you subject to ordinances,
 (Touch not; taste not; handle not;
 Which all are to perish with the using;) after the commandments and doctrines of men?
 Which things have indeed a show of wisdom in will worship, and humility, and neglecting of the body: not in any honor to the satisfying of the flesh.
American King James Version×). Another form of penance is trying to do enough good deeds to outweigh and make up for one's sin. Both imply that we must pay for our own sins—but none of us could ever do enough to pay for even one sin.
All sins are evil because they violate the perfect, loving laws of our Creator God. The only sacrifice great enough to pay for the guilt of our sins has already been offered—the suffering and death of the Son of God, Jesus Christ. So doing penance is a gross insult to God. It implies that Jesus' sacrifice and God's grace are unnecessary.
And what is God's "grace"? It is the totality of all the gracious gifts that God offers to mankind. And among the greatest of those gifts is the complete forgiveness of all past sins. That, in turn, opens the door to all the rest of God's gifts, including the ultimate gift of eternal life (Romans 6:23 Romans 6:23For the wages of sin is death; but the gift of God is eternal life through Jesus Christ our Lord.
American King James Version×). "For by grace are ye saved through faith; and that not of yourselves: it is the gift of God" (Ephesians 2:8 Ephesians 2:8For by grace are you saved through faith; and that not of yourselves: it is the gift of God:
American King James Version×, King James Version).
But don't confuse grace with its counterfeit—"cheap grace." Grace means forgiveness of past sins, not permission to continue sinning, as Satan would like for us to think. Jude warned of "godless men, who change [pervert] the grace of our God into a license for immorality" (Jude 4, NIV; compare Romans 6:1-2 Romans 6:1-2  What shall we say then? Shall we continue in sin, that grace may abound?
 God forbid. How shall we, that are dead to sin, live any longer therein?
American King James Version×).
Grace is made possible by Christ's sacrifice and by Him serving as our merciful Mediator, Advocate and High Priest (1 Timothy 2:5 1 Timothy 2:5For there is one God, and one mediator between God and men, the man Christ Jesus;
American King James Version×; 1 John 2:1-2 1 John 2:1-2  My little children, these things write I to you, that you sin not. And if any man sin, we have an advocate with the Father, Jesus Christ the righteous:  And he is the propitiation for our sins: and not for our's only, but also for the sins of the whole world.
American King James Version×). "Seeing then that we have a great High Priest ... let us therefore come boldly to the throne of grace, that we may obtain mercy and find grace to help in time of need" (Hebrews 4:14-16 Hebrews 4:14-16  Seeing then that we have a great high priest, that is passed into the heavens, Jesus the Son of God, let us hold fast our profession.  For we have not an high priest which cannot be touched with the feeling of our infirmities; but was in all points tempted like as we are, yet without sin.  Let us therefore come boldly to the throne of grace, that we may obtain mercy, and find grace to help in time of need.
American King James Version×).