A passage from Pieter's book "Grace Reign"

Making the grace of Christ void

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Galatians 3: 12:

“And the law is not of faith: but, the man that doeth them shall live in them”.

“Note 17 at Gal. 3:12: What a radical statement! "The law is not of faith." When combined with Romans 14:23 which says, ". . .whatsoever is not of faith is sin," then this statement must have been near blasphemy to the legalistic Jews, just as it is to legalistic Christians today. It is sin for the New Testament believer to try to relate to God on the basis of attempting to keep the Old Testament law. The law itself is not sin (Rom. 7:7). But it is sin to try to use the law for justification with God. That was never the purpose of the law (see ref. k at v. 11, p. 1066).” “Under the law a person got what he deserved. Under faith, the New Testament believer receives grace because of what Jesus did for him. Any departure from faith, especially a departure back to the Old Testament law, does not bring a curse on your life, but voids the work of Christ and is the worst sin of all.”

I do not set aside the grace of God; for if righteousness comes through the law, then Christ died in vain.”
Gal 2: 21

The reason Paul had to write letters to the churches was because legalistic people came among his followers. Whether you are a Jew or Gentile, if you are ignorant of grace, you make the power of Jesus of no effect by the law.

You have become estranged from Christ, you who attempt to be justified by law; you have fallen from grace.
Gal 4: 5

The JB Phillips Translation gives a better explanation:

If you try to be justified by the Law you automatically cut yourself off from the power of Christ, you put yourself outside the range of his grace.

The purpose of the law is simple:

Now we know that whatever the law says, it says to those who are under the law, so that every mouth may be silenced and the whole world (Jew, Gentile) held accountable to God.
Rom 3: 19 emphasis mine, brackets mine

The purpose is that every mouth be stopped and that nobody, and that includes the whole world (Gentile and Jew), has an excuse. The law gives you knowledge of sin and makes you guilty before God.

Therefore Paul comes to this conclusion:

For all have sinned and fall short of the glory of God!
Rom 3: 23

Through his letters Paul warns his followers (Jew and Gentile) to be careful of the law because it creates a sin conscience. Once they have a sin conscience there comes a veil over the heart. In other words the work of Christ in our hearts has been made void or has become of no effect.

But even to this day, when Moses is read, a veil lies on their heart.
2Cor 3: 15

Then they begin serving God through the soul in subject to self-effort and not grace. This is what separates them from the spirit within. The law works on the flesh, and grace through the spirit. We serve God through our spirits. It does not matter if you are Gentile or Jew, African or Westerner; the law can pollute your conscience if you become ignorant of grace. Today, the church all over the world has been polluted by laws that make the grace of Jesus void in the hearts of believers. There are frustrations because they do not experience the life of Christ in their lives. They believe in Jesus, they believe He can do anything, but they have set grace aside and their faith is made void and the promise made of no effect. The promises come by grace through faith and not by works of the law.

But now the righteousness of God apart from the law is revealed, being witnessed by the Law and the Prophets, even the righteousness of God, through faith in Jesus Christ, to all and on all who believe. For there is no difference; for all have sinned and fall short of the glory of God, being justified freely by His grace through the redemption that is in Christ Jesus, whom God set forth as a propitiation by His blood
Rom 3: 21 – 25emphasis mine

For Christ is the end of the law for righteousness to everyone who believes
Rom 10: 4

What purpose then does the law serve? It was added because of transgressions, till the Seed (Jesus Christ) should come to whom the promise was made; and it was appointed through angels by the hand of a mediator. But the Scripture has confined all under sin, that the promise by faith in Jesus Christ might be given to those who believe. But before faith came, we were kept under guard by the law, kept for the faith which would afterward be revealed. Therefore the law was our tutor to bring us to Christ, that we might be justified by faith. But after faith has come, we are no longer under a tutor.
Gal 3: 19, 22 – 25 NKJV Brackets Mine
The law - legalism and works – will keep on showing you that you come short until you fall on your knees and call for mercy. As a believer who is influenced by the law, you have set aside the grace of Jesus and you will get so frustrated that you eventually call out for help. It is like a tutor that brings you to Christ Jesus our Savior. If you are born again, then you are a child of God in Christ Jesus, the Author of life. The relationship under the new covenant is for us to abide in Him and for Him to abide in us. The new covenant is established in the blood of Jesus Christ, which cleanses us from all sin, so that God can live in us. The question arises: how does Satan tempt the children of God?