What, then, does Paul mean by saying, "by the works of the law shall no flesh be justified (attain the standing of righteousness in the sight of God)"?

Paul means now that the righteousness of God has been revealed in the death and resurrection of Christ, we no longer are to return to an inferior covenant, the Law of Moses, making the cross of Christ an unnecessary sacrifice on God's part.

Paul does not mean that under the old covenant no person was able to please God, because that would contradict Luke 1:6.

Let us offer the following paraphrase of verses seventeen and eighteen of the second chapter of Galatians, in the light of Paul's attack on the doctrine of the Judaizers who were attempting to persuade the Galatian Christians to be circumcised and to adopt other aspects of the Law of Moses as part of their salvation in Christ:

"If in the process of seeking to attain right standing in the sight of God through faith in Christ, we sin, does that mean that Christ endorses sin?

"God forbid that such a thing should be.

"If I return to a life of sin, building again the life of the natural man, I prove that I am a transgressor, a lawless person.

"It is true, rather, that I am dead to the Law of Moses, because the Law killed me. My death releases me to be raised from the dead in Christ so that I may live to God, not being obligated to attempt to please God by the laws to which I now am dead.

"Therefore Christ is not causing me to sin. Rather, He has enabled me to lay aside the Mosaic statutes and turn my attention to God."

Continuing with verse 20:

"Circumcision, the Ten Commandments, the laws of foods and the observances of days, no longer apply to me. I am crucified with Christ. As soon as a person dies the Law of Moses no longer applies. Circumcision, the Ten Commandments, the laws of foods and of religious days, have no authority over a dead person.

"The Law governs the living, it does not govern the dead. After death there follows resurrection and judgment, not a continuation of the requirements of the Law of Moses.

"I did not come back to life, bringing myself back under the Law of Moses. It is true that I am alive, but in actual fact it is not I who am living. It is Christ who is living in me. He did keep the Law of Moses perfectly, and then died so that the righteousness He earned under the Law might be applied to me, who was not able to keep the Law.

"Since my new life is Christ, and Christ observed the Law perfectly, I am righteous in Him.

"Christ loved me and gave Himself for me. Therefore I live by believing in Him, trusting in Him, relying on Him for every detail of my existence. Christ is my life. I am an eternal part of Him. For me to live is Christ.

"The Law of Moses no longer has any application to my life, any power to condemn me."

What is an individual doing when he attempts to gain righteousness by observing all or part of the Law of Moses?

He is setting aside the grace of God. If it is possible to attain right standing in the sight of God by observing the Law of Moses, it was not necessary for Christ to endure the agony of Gethsemane and of the cross of Calvary. His suffering was not needed. (from A Study Guide for the Book of Galatians)