Archibald Alexander: Suffering the penalty is only half the requirement for justification, and Christ satisfied all

To suppose that suffering the penalty is an equivalent for obedience, and entitles to the same rewards is extremely absurd. It would be to suppose that Jehovah who loveth righteousness and hateth iniquity, would be as well pleased with sin, accompanied with its due punishment, as with perfect obedience to his own most holy law. The enduring a penalty in his own person, or by another, never can entitle any one to any thing else than exemption from that which he has already endured. …

If a surety would secure the inheritance for him, he must obey the law in his stead, as well as suffer its penalty. Hence it appears evident, that justification includes more than merely the remission of sins, or it would be no justification; and although pardon is included in justification; yet the transaction receives this denomination not from the forgiveness of sin, but from the imputation of righteousness, by which the believer is constituted righteous; and by which a title to eternal life is procured for him by the merit of his surety.

Justification, therefore, is not merely the forgiveness of sin, but in addition to this, a declaration that the justified person has a right to the blessings promised. He not only obtains deliverance from the sentence of condemnation, but instantly is constituted an heir of God, a joint-heir with Christ in the heavenly inheritance.

-Archibald Alexander, “A Treatise on Justification by Faith”