Matthew 26:28 For this is my blood of the new testament, which is shed for many for the remission of sins.
Remission very simply is defined as the cancellation of a debt or a charge or a penalty. It can mean pardon or forgiveness. There is much more in what Christ did at His cross, but we will think of that remission in these words. In Matthew 26:28 the scriptures give us the word remission. This comes from the Greek work aphesis. This is defined as deliverance, forgiveness, liberty, etc. This would fit with our dictionary definition of remission. It was the blood that would bring forgiveness, deliverance, and liberty. This shows that forgiveness, deliverance, and liberty did not exist until the blood of the new testament came. This fact helps us to transition into the next portion of Romans 3:25.
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;
Here in Romans 3:25 we also see the word remission; for the remission of sins that are past. The word remission here is not the same word we find in Matthew 26:28. Here the word is paresis. This word carries with it the meaning of passing over or neglecting/disregarding. What it means is for those sins of the past, God would pass over them, not dealing justly with them for the time being. He would tolerate their existence for a time. God would pass over these sins that were past. Being past, what were they before? They were before the propitiation that was mentioned earlier in this verse. God would patiently wait to deal with the sins of the past, those prior to the shed blood of Jesus Christ, until the time of the propitiation. God could not be just and not deal with sins. He would deal with these sins but it would be in the body and soul of Jesus Christ, our propitiation. All from the Old Testament for whom Christ would die would have their sin dealt with in this way. There was a suspension of judgment until the time the propitiation would be made. This declared the righteousness of God in that He would not let sin go unpunished. He would deal with the sin of all His people both before and after Christ in the sacrifice that was the cross. The faith, the gospel of Christ would be established in blood.
Exodus 34: 6 And the Lord passed by before him, and proclaimed, The Lord, The Lord God, merciful and gracious, longsuffering, and abundant in goodness and truth,
The longsuffering of God shows us how He dealt with sin all at once. There was no individual propitiation. Christ would shed His blood once for all.
Hebrews 10:10 By the which will we are sanctified through the offering of the body of Jesus Christ once for all.
This once for all propitiation satisfied a holy God who required nothing else be added to this finished work of Christ. By this finished work we are redeemed, forgiven, sanctified, declared righteous, and therefore justified in the eyes of God.