John Gill on the nature of pardon and justification

Pardon is of men that are sinners, and who remain such, and may be called so, though pardoned sinners; but justification is a pronouncing persons righteous, as if they had never sinned; it is one thing for a man to be arraigned at the bar as a criminal, and be tried, cast, and condemned, and after that be pardoned; and another thing for a man to be tried by law, and to be found and declared righteous by it, as though he had not transgressed it.

… Pardon takes away sin from the sinner, but does not give him a righteousness, as justification does; pardon takes away the filthy garments; but it is justification that clothes with change of raiment, with the robe of Christ’s righteousness; these are two distinct things (Zech. 3:4).

… The righteousness of Christ, by which men are justified, is the fulfilling of the law; Christ came to fulfil it in the room of his people; and he is the fulfilling end of it to them, for righteousness; which is inherent in him, the author of it: not so pardon; that does not fulfill the law, gives no righteousness; nor does it reside in Christ, as righteousness does (Rom. 10:4; Isa. 45:24).

… Pardon lies in the non-imputation of sin; justification in the imputation of righteousness: righteousness is imputed, but pardon is not (Rom. 4:6,7).

(John Gill, A Body of Doctrinal Divinity)