Romans 3:25b

God presented him as a sacrifice of atonement, through faith in his blood. He did this to demonstrate his justice, because in his forbearance he had left the sins committed beforehand unpunished—

(v25, NIV)

What does it mean that he left previous sins unpunished? Well, what do we deserve when we sin? We deserve death for when we sin. However, if everyone sins, why isn't everyone dead? If I sin, I should be instantly dead as penalty for that sin. Instead, God has left these sins which I have committed unpunished. He is patiently in kindness postponing his wrath upon us. The punishment we deserve for the sin we commit is being stored away and withheld from us for the time being. This is what God does in his kindness and we should be thankful to him for that.