whom God put forward as a propitiation by his blood, to be received by faith. This was to show God's righteousness, because in his divine forbearance he had passed over former sins.  It was to show his righteousness at the present time, so that he might be just and the justifier of the one who has faith in Jesus.
(Romans 3:25-26 ESV)
In America, we take great pride in the justice of our judicial system. We want judges to preside over cases, treating both sides with fairness and equity. We don't want judges to decide the outcome ahead of time, but judge on the facts of the case - or lead a jury to do so.
Imagine, however, if there was a judge that continually let guilty people walk out of the courtroom as free men. I don't mean controversial cases where there's uncertainty about guilt or innocence, but cases where guilt is absolutely clear. Yet, the judge keeps letting them walk free, declaring their guilt and not giving them the sentence they deserve. The public outcry would be loud against the lack of justice in the judge's courtroom.
In Romans, Paul is saying we should have similar feelings after reading the Old Testament. Romans 3:25 says that God, "in his divine forbearance, he had passed over former sins." In other words, there were sins committed before the days of Jesus, for which God had not held people accountable. That's what "passed over" means. He didn't give them what they deserved for their sins.
For example, in 2 Samuel, King David murders Uriah and takes his wife as his own. If there's a sin worthy of the death penalty, that's it. Yet, David lives on as King of Israel. He faces consequences and punishment for his sin, but he's not stoned like the man who gathers sticks on the Sabbath in Numbers 15:32ff. David's punishment was not justice. God passed over his sin.
How can a perfectly just judge, pass over sins, not giving people what they deserve? According to Romans 3:25-26, we have to look at Jesus for the answer to that question. God put Jesus forward as a propitiation by his blood. That means Jesus was given as a sacrifice that appeased the wrath of God. So, by putting Christ forward as a propitiation it showed God's righteousness. It showed that he will not leave sin unpunished. How? By providing Jesus as a substitute for the punishment that His people deserve.
According to Romans 3:26, this allows God to be both just (punish sins as they deserve) and the justifier (one who makes someone righteous) of those who have faith in Christ. In other words, for God to declare guilty people righteous and not punish them would be unjust. But, because Christ's death on the cross satisfied the wrath of God, he can remain just and justify his people at the same time.
The only way any of this grace and forgiveness is possible is by Jesus taking on flesh and dying on the cross for sinners. Because of Jesus, God remains just and we are able to be forgiven. This is at the very core of what we should meditate on this Christmas.