Romans 3:25c

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)

So God put forth Jesus as a sacrifice of atonement. Apparently, this was done to demonstrate his justice. How is this in any way a demonstration of his justice? It is somewhat linked to the fact that God has passed over previously committed sins. Well, when God leaves sin unpunished, he is unjust. It is unjust to not give the proper punishment for wrongdoing. However, God does this when he chooses to leave previously committed sins unpunished. In this situation, what must God do? God must deal out the proper punishment where punishment is due. One way God could do this is to send all us sinners to hell. Hell is what we deserve for our life of sin and continual trading away of his glory. God could do that and he would be perfectly just in doing that. However, by sending his son as an atoning sacrifice, God also satisfies his perfect justice. Payment is demanded for the sins previously committed and Jesus is the perfect sacrifice. He lived a perfect life and is more than able to pay the debt we owe. In this sacrifice, God's wrath against us for our sins is satisfied. God is just because proper payment is made for the sins committed.