Faith is the one thing that distinguishes those seperated from God and those who are His children. Everyone has been united in their guilt before God. God gave us the law to show how far short we fall from what He expects. The law outlines what a "righteous" life looks like. As we look at the list, it names and illustrates the various ways that our sin is displayed. It eliminates our ability to lay claim to favor with God due to our heritage, words, or deeds.
Paul is very clear in this matter. We are all united in our wickedness. To demonstrate this, he ties together various Old Testament quotes (vs. 10-18) that show how we have failed in our character, conversation and conduct. In this way, we all stand accountable to God. The law demonstrates our guilt, our wickedness, our helplessness, our need to be rescued from ourselves.
While the law has united us in guilt, it allows God to offer another way to attain the righteousness that He desires. Since we cannot achieve that through the law, He opens another way apart from the law. Instead of obedience to the law, now we are made righteous through our faith in the One who kept the law and then died in our place. Jesus died to take the punishment we deserved for breaking the law.
Righteousness comes from God. It is not something we are capable of attaining, so He provided a way that He could give it to us. It comes to us through faith. We are to simply believe that we are all united in our guilt as sinners. Because of this, we deserve God's judgement. But God has provided a way to escape His wrath by freely giving to us grace. Grace is an undeserved gift; something that we cannot earn and can never claim to deserve. Grace is a gift of God made possible by the death of Jesus.
If God simply overlooked our sin, He would be unjust. This would require Him to act against His nature and therefore, would require Him to sin. Instead of overlooking our sin, He punished it thoroughly, but not by punishing us as we deserved. Rather than punishing us, Jesus stepped in and took our punishment for us. There on the cross, Jesus became our sacrifice. His life was offered in our place, to pay for our sins. Now God remains just because the debt of our sin was paid for. At the same time, He is the one who justifies us through faith in Jesus. He upholds His justice and still provides a way for our salvation.
The law does its perfect work in uniting us under sin. Jesus does His perfect work in liberating us from sin. Now, we are able to stand righteous before God, not because we have earned it or deserve it. Rather, it is through the grace provided through Jesus.
We are justified by faith, not by observing the law.