Why are sins called debts?

“All have sinned and fall short of the glory of God” (Rom 3:23; see Q168, Q187): what is due to God (Rev 4:11) has not in fact been rendered. We are like servants unable to pay an impossibly great debt, as the Lord teaches in the parable of the unforgiving servant: the king forgave his slave a debt of ten thousand talents; but that slave, going out, seized a fellow slave who owed him a hundred denarii… “You wicked slave! I forgave you all that debt because you begged me: should you not also have had mercy on your fellow slave, as I had mercy on you?… So also my heavenly Father will do to you, if you do not each forgive your brother from your heart” (Mat 18:21–35).