Albert Barnes on Romans 4:8

Will not impute sin. On whom the Lord will not charge his sins; or who shall not be reckoned or regarded as guilty. This shows clearly what the apostle meant by imputing faith without works. It is to pardon sin, and to treat with favour; not to reckon or charge a man's sin to him; but to treat him, though personally undeserving and ungodly, (Rom 4:5) as though the sin had not been committed. The word "impute" here is used in its natural and appropriate sense, as denoting to charge on man that which properly belongs to him.