Q. 88. Of how many parts doth the true conversion of man consist?
Of two parts; of the mortification of the old, and the quickening of the new man.
 Romans 6:4–6; Ephesians 4:22–23; Colossians 3:5; 1 Corinthians 5:7.
Q. 89. What is the mortification of the old man?
It is a sincere sorrow of heart, that we have provoked God by our sins; and more and more to hate and flee from them.
 Psalm 51:3, 8, 17; Luke 15:18; Romans 8:13; Joel 1:12–13.
Q. 90. What is the quickening of the new man?
It is a sincere joy of heart in God, through Christ, and with love and delight to live according to the will of God in all good works.
 Romans 5:1–2 and 14:17; Isaiah 57:15;  Romans 6:10–11; 1 Peter 4:2; Galatians 2:20.
Q. 91. But what are good works?
Only those which proceed from a true faith, are performed according to the law of God, and to His glory; and not such as are founded on our imaginations, or the institutions of men.
 Romans 14:23;  1 Samuel 15:22; Ephesians 2:2, 10;  1 Corinthians 10:31;  Deuteronomy 12:32; Ezekiel 20:18; Matthew 15:9.
Conversion is customarily spoken of as having two parts: repentance and faith. Repentance is a turning away from sin, faith is a turning away to God. But faith and repentance are really volitional responses due to changes in our inner man, which are wrought by the Holy Spirit who works in us both to will and to do of His good pleasure (Phil 2:13). These inward changes may best be described as in Q/A 88, as having two parts: mortification of the old man and quickening of the new man.
Now, we must not think that the regenerate person has two natures that are independently warring against one another (as some imagine passages, such as Romans 7:23, are teaching). No, the Apostle Paul teaches us that “our old man is crucified with [Christ], that the body of sin might be destroyed, that henceforth we should not serve sin” (Rom 6:6). And he says: “Therefore if any man be in Christ, he is a new creature: old things are passed away; behold, all things are become new” (2 Cor 5:17). These verses clearly teach us that regeneration involves a renovation of our nature rather than an implantation of a new nature. This new nature, however, is not completely void of corruption. But daily, by the power of the Holy Spirit, our old man or old inclination to sin is put to death (mortified), and our new man, or new desires after Christ are exercised. So more and more, we hate sin and are grieved when we know God is displeased with our deeds, words or thoughts. At the same time, more and more we deepen in our love for God and His Word, and more and more we desire to please God and to conform to the image of Christ. This is the process of sanctification.
The work of sanctification, moreover, manifests itself in repentance, faith and good works. Good works are not simply what appear to be good in the sight of man. In the sight of God, no unregenerate persons can ever do any good work, for all his righteousness would be tainted with sin and are as filthy rags before God (Isa 64:6). A regenerate person can have a true desire to do good works or, in other words, a true desire to glorify God by what he does. And although his actions can never be perfectly pure in this life (i.e., his motivation will have a mixture of godly desires and selfish designs, however minute the traces of it), his deeds are by grace acceptable to God on account of the righteousness of Christ imputed to him, if they are done by faith.
Heidelberg Catechism >