We all know what justice is. We can see it on TV. Every police procedural show ends up with the bad guy getting caught. They offenders go to prison. The criminals get punished. That's justice. Justice for sinners is terrible. Every sin means we have rejected God. Justice demands nothing but temporal and eternal punishment.
You might not know justification, though. It is like forgiveness. It's when the guilty are declared innocent. The offenders are freed. The criminals are not punished. Justification for sinners is wonderful. Despite our sin, God declares us innocent and gives us eternal life.
The problem, however, is that justice and justification are opposites. One cannot be fully just and the one who justifies. But that is what St. Paul says about God. How is this possible? Only through the incarnation of Jesus Christ, God becoming man. He is the key to both God's justice and his justification.