Therefore [dioti] by [ek] the deeds [ergon] of the law [nomos] there
shall [dikaioo] no [pas][ou] flesh [sarx] be justified [dikaioo] in
his [autos] sight [enopion]: for [gar] by [dia] the law [nomos] is the
knowledge [epignosis] of sin [hamartia].KJV-Interlinear
20 For by
works of the law no human being will be justified in his sight, since through
the law comes knowledge of sin. ESV
The Law is a rule of conduct, mental, physical, moral and such, as prescribed by a higher authority. In other words, the Law, which Moses wrote out a long time ago, set forth a standard of perfection, which in itself, made it quite clear, that no person is capable of keeping.
To keep the Law would make a person in effect equal or on the same level of perfection as God, who is perfect.
And in order to have a relationship with God, a person has to possess that same level of perfection, otherwise, no relationship is possible. The perfect cannot relate to the imperfect.
So, if we are imperfect and incapable of attaining perfection, then how can we possibly achieve any relationship with God?
And the answer is of course, we need someone who is perfect, as well as someone who is already on the same level as God, to step in and take our place, and help us get to that perfect level.
The person is of course Jesus Christ.
But that concept is not addressed in this verse.
What we learn here is that the Law gives us the necessary information so that we can learn of our flaws. And once we know our weaknesses, then we can learn of what we need, namely a savior.
The imperfect, has no future, no eternal life. So we need eternal life in order to have a future. Otherwise we are delegated into oblivion forever.
So, God set out a plan by which He would do what was necessary to save us, and also by giving each person the choice as to whether they want the benefits of that work. We believe in Christ and that is our acceptance of His work.
In so believing in Him, we reject our works, we reject our efforts to rise to Gods level on our own.
The Law is the standard that indicates that we are weak and need help.
This world is ruthless and in order to get by, people have to resort to lying cheating, compromising their morals, just to survive.
Some people do this because they are evil. But most folks are not bad per se, but just have no choice in the matter, and circumstances drive the behavior.
But in Christ, we have an out, from this world, and a better future.
Therefore, do not fix your attention on your flaws, do not try to substitute your efforts and morals in place of Gods work and grace, and instead, conduct yourself in a daily study, learn doctrine and grow up in grace and in so doing you will replace the standards of this world, with the standards of God, namely the spiritual resources of the spiritual life.