Romans 3:23-26 (NIV)
For all have sinned and fall short of the glory of God, and are justified freely by his grace through the redemption that came by Christ Jesus. 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--he did it to demonstrate his justice at the present time, so as to be just and the one who justifies those who have faith in Jesus.
There verses are packed with the essence of Christianity. First, all we are all sinners. The Bible uses a Greek word to mean falling short of the mark. Second, Jesus Christ became a blood sacrifice of atonement. A sacrifice is like a payment. God has no need of anything so what else could be offer other than a sacrifice? Atonement is a big word which means satisfaction. Jesus' sacrifice was made to satisfy God's sense of justice. Third, it is by faith in the atoning power of this sacrifice that we have redemption. That's another fancy word which means to buy back. The effect of redemption is forgiveness of sins.
Finally God's justice is what is being displayed here. Socrates once said "It may be that Deity can forgive sins, but I don't see how." He understood the fundamental problem: how can perfection accept imperfection? This is the wonder of the last part of this verse. God is both just and the one who justifies. Picture a judge sitting behind his desk pronouncing a guilty verdict, and then requiring a death penalty to satisfy justice. Then, the judge stands up, removes his robe walks around to stand next to the sinner and announces "But I will satisfy justice by suffering the penalty myself."