If Christians are justified by grace through faith, and therefore forgiven of all their sins,—past, present and future,—why does the Word of God teaches us that if we regard iniquity in our heart, the Lord will not hear us (Ps 66:18), and that we are to pray for forgiveness (e.g., Mt 6:12)?

First of all, it must be stated that Christians are indeed forgiven or pardoned on account of Christ’s substitutionary and propitiatory death. His death is substitu-tionary in that He was punished on our behalf; it is propitiatory in that it averted the wrath of God that was due to us on account of our sin. Thus, Christians may not expect any punishments according to what our sins deserve, which punishments if meted out would be no less than eternal torment and separation from God, as only such a punishment is fitting for rebellion against an infinite God. It is because Christ took upon Himself the wrath of God due to His elect for their sin, that Christians may be adopted as children of God.

Now, just as a father does not disown his children when they disobey him, God does not disown His children when they sin against Him. But just as a child incurs his father’s displeasure and sometimes chastisement due to disobedience, so our sin incurs our Heavenly Father’s displeasure and sometimes chastise-ment (Heb 12:6–10). Thus our Heavenly Father proclaims: "If his [i.e., Christ, as prefigured by David] children forsake my law, and walk not in my judgments; If they break my statutes, and keep not my commandments; Then will I visit their transgression with the rod, and their iniquity with stripes. Nevertheless my lovingkindness will I not utterly take from him, nor suffer my faithfulness to fail" (Ps 89:30–33).

In the same way, although the child of God can come to the Heavenly Father in the righteousness of Christ, his prayers may not always be heard and answered. This is why the Psalmist say, "If I regard iniquity in my heart, the Lord will not hear me" (Ps 66:18). This verse is no doubt applicable to every believer, since David (probably the inspired author) was certainly a believer. Indeed, the remnant of corruption in the regenerate makes them likely candidates for regarding iniquity in the heart. The Apostle John expresses the same doctrine positively: "And whatsoever we ask, we receive of him, because we keep his commandments, and do those things that are pleasing in his sight" (1 Jn 3:22). Just as a kind father does not respond indulgently to the requests of a disobedient and unrepentant child, so our Heavenly Father does not hear us when we sin against Him and fail to repent of them.

For all these reasons, it is important that when we pray we should seek God’s fatherly forgiveness though we have been legally forgiven and will not experience His eternal wrath.

When we ask God to forgive us for our sins, we are: (1) expressing our remorse and repentance for our sins; (2) pleading with Him not to chastise us according as our deeds deserve; (3) requesting Him to restore unto us the joy of His salvation (Ps 51:12) or, in other words, to grant us a renewed sense of pardon that we have lost when we sinned against Him.

We must warn, however, that: "He that turneth away his ear from hearing the law, even His prayer shall be abomination" (Prov 28:9). Ultimately, this verse finds full expression in the unregenerate who may be a deluded believer, for a person who persistently and unrepentantly disregard God’s law, can hardly be a Christian. Calvin says it well: "It is right that they who bar their hearts should find God’s ears closed, and… they who by their hard-heartedness provoke his severity should not feel Him conciliatory" (ICR 3.20.7).