Whose righteousness is declared for the remission of sins?

Christ’s righteousness is the righteousness declared for the remission of sins. The context explains that Christ, the propitiator, becomes the propitiation through faith in His blood, and thereby His righteousness—not ours—is declared for the remission of sins that are passed through God’s forbearance.

Les teaches that Christ Jesus is the propitiation, and that this word is the most inclusive word in the New Testament. He explains that Christ becomes our propitiation by faith in His blood, not by His life or His three years of miracle working. Through that faith, He declares His righteousness, not yours or mine, for the remission of sins that are passed through the forbearance of God. Verse 26 again declares His righteousness—whose? Christ, the propitiator—so that He might be just and fair, and the justifier of him that believeth.

Romans 3:25

Whom God hath set forth to be a propitiation through faith in his blood, to declare his righteousness for the remission of sins that are past, through the forbearance of God;

What does it mean that God declares His righteousness for the remission of sins?

God declares His righteousness by showing Himself just and fair while justifying sinners through faith in Christ's blood. The remission of sins rests on Christ's finished work, not human works or commandments, so no one can boast; the law of faith calls people simply to believe the Gospel.