J.I. Packer: The penal substitutionary view of the atonement is the foundation of Christ's righteousness imputed to the believer in justification

"Salvation in the Bible is by substitution and exchange: the imputing of men’s sins to Christ, and the imputing of Christ’s righteousness to sinners. By this means, the law, and the God whose law it is, are satisfied, and the guilty are justly declare immune from punishment. Justice is done, and mercy is made triumphant in the doing of it. The imputing of righteousness to sinners in justification, and the imputing of their sins to Christ on Calvary, thus belong together; and if, in the manner of so much modern Protestantism, the penal interpretation of the Cross is rejected, then there is no ground on which the imputing of righteousness can rest." (J.I. Packer, Introductory Essay in James Buchanan’s “Justification,” published by Banner of Truth, p. xiii)