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; Whom God hath set forth - Appointed and published to be a propitiation, ιλαστηριον, the mercy-seat, or place of atonement; because the blood of the sacrifice was sprinkled on and before that, in order to obtain remission of sin, punishment, etc. The mercy-seat was the lid or cover of the ark of the covenant, where God was manifest in the symbol of his presence, between the cherubim; therefore the atonement that was made in this place was properly made to God himself. See the note on Luke 18:13.
Through faith in his blood - This shows what we are to understand both by the απολυτρωσις, redemption, and the ιλαστηριον, propitiation; viz. that they refer to the sacrificial death of Jesus Christ, as the atonement made, and the price paid down, for the redemption of the souls of men.
To declare his righteousness - εις ενδειξις, for the manifestation of his righteousness; his mercy in saving sinners, by sending Jesus Christ to make an atonement for them; thereby declaring his readiness to remit all past transgressions committed both by Jews and Gentiles, during the time in which his merciful forbearance was exercised towards the world; and this applies to all who hear the Gospel now: to them is freely offered remission of all past sins.