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;
Note 9 at Ro 3:25: This verse is speaking of the sins that were committed under the Old Covenant, before the sacrifice of Jesus was made. Those sins were also paid for by the blood of Jesus. The Old Testament sacrifices were only types and shadows of the true sin offering that Jesus made. It was impossible for the blood of bulls and of goats to ever take away sins (Heb 10:4).
The Lord dealt with sins under the Old Testament through His forbearance. In a similar way that a person gives a check or credit card in exchange for the real currency desired, so the Lord gave the Old Testament sacrifices. However, just as these substitutes would be unacceptable if there was no reality to back them up, so the Old Testament sacrifices only served as a token of the real sacrifice of Jesus that would pay for sin (Heb 9:13-14).