“I, even I, am he that blotteth out thy transgressions for mine own sake, and will not remember thy sins.”
“I, even I am he, that blotteth out thy transgressions for mine own sake,…. The same with “sins” in the next clause; original sin, and actual sins; which are transgressions of the law of God, of which the law accuses, for which it pronounces guilty, curses, and condemns; which are contrary to the nature of God, strike at his deity, and must be abominable to him; they are many, yea infinite, and yet all pardoned for Christ’s sake; which is here expressed by a “blotting” them out, in allusion to the blotting of a debt book: sins are debts, and these are many, and which cannot be paid by the sinner; Christ has made full payment; as the surety of his people: upon this the debt book is crossed; these debts are remitted for his sake: or as a cloud is blotted out, dispelled by the wind, or scattered by the sun; see Isaiah 44:22, so as to be seen no more with the eye of avenging justice, or to be charged against the sinner to his condemnation.
The author of this blessing of grace is the Lord, “I, even I am he”; who had been so ill used, and maltreated, as before declared; whose law had been broken in such a manner; and who is the Lawgiver that is able to save and to destroy; and who hates and abhors sin, and is strictly just; and yet, notwithstanding all this, forgives it; and which he repeats for the confirmation of it, and seems to express it with the utmost pleasure, and as glorying in it, and as if it was an honour to him, and a jewel in his crown; and indeed it is his sole prerogative; none can forgive sins but him: and this he does for his own sake; it is not procured by anything of the creature; not by riches, nor by righteousness, nor by repentance, nor by faith, nor by obedience to any ordinance; it is not for the sake of these that the Lord forgives sin, but for his own sake, and his Son’s sake, which is the same; it is an instance of unmerited and distinguishing grace; it flows from the free grace of God; it is a branch of the covenant of grace; it is through the blood of Christ, and yet according to the riches of grace; and it is for the glory of all the divine perfections, justice, truth, and faithfulness, as well as grace and mercy; and after such a list of sins of omission and commission, to hear such language as this is surprising grace indeed! and will not remember thy sins; God forgives and forgets; God will not remember the sins of his people against them; having forgiven them, he will never punish them for them, which is meant by remembering them; see Jeremiah 14:10.”
John Gills Exposition of the Bible Commentary