When we stand before God we will be judged by our outward obedience to the law of God, and by our inward adherence to the law.
If by chance one were to be found without fault in these two areas, all will be found out in the third. Because God will lastly judge the secrets of each person’s heart.
What will God find when He searches your heart? YOU DO NOT KNOW. Because the secrets of a person’s heart are a mystery even to himself.
Only God knows the true thoughts of the heart. This is why we dare not place our trust in our outward obedience or inward adherence to God’s law. We dare not let hubris cause us to overestimate our righteousness.
This is the argument Paul is setting up in the verses before us: To help us understand that we do not measure up, and even if we did we would have no way of knowing.
Thus every person should depend on the grace of God alone, whether apart from the law or under the law.