The seriousness of NT believers returning to the Law is shown through several verses in Scripture. Here are just two of them in both the ESV and the KJV.
Fallen From Grace
Galatians 5:4 You are severed from Christ, you who would be justified by the law; you have fallen away from grace. ESV
Galatians 5:4 (KJV) Christ is become of no effect unto you, whosoever of you are justified by the law; ye are fallen from grace.
This verse is pretty clear that you have fallen from Grace if you think the law will justify you. What does justified mean? To be counted righteous. If you feel that being obedient to the law makes you more righteous, than you have fallen from grace.
Because see then your Salvation becomes Jesus plus the Law and not Christ alone. Following the Law is a work of the flesh and no one can follow the Law. Only Christ successfully followed the Law and thus He fulfilled it for each of us.
This commentary by Luther is quite helpful to understanding this passage.
Galatians 5:4 (Underlining added)
Paul in this verse discloses that he is not speaking so much of circumcision as the trust which men repose in the outward act. We can hear him say: “I do not condemn the Law in itself; what I condemn is that men seek to be justified by the Law, as if Christ were still to come, or as if He alone were unable to justify sinners. It is this that I condemn, because it makes Christ of no effect. It makes you void of Christ so that Christ is not in you, nor can you be partakers of the knowledge, the spirit, the fellowship, the liberty, the life, or the achievements of Christ. You are completely separated from Him, so much so that He has nothing to do with you any more, or for that matter you with Him.” Can anything worse be said against the Law? If you think Christ and the Law can dwell together in your heart, you may be sure that Christ dwells not in your heart. For if Christ is in your heart He neither condemns you, nor does He ever bid you to trust in your own good works. If you know Christ at all, you know that good works do not serve unto righteousness, nor evil works unto condemnation. I do not want to withhold from good works their due praise, nor do I wish to encourage evil works. But when it comes to justification, I say, we must concentrate upon Christ alone, or else we make Him non-effective. You must choose between Christ and the righteousness of the Law. If you choose Christ you are righteous before God. If you stick to the Law, Christ is of no use to you.
(from Luther’s Commentary on Galatians, PC Study Bible formatted electronic database Copyright © 1999, 2003, 2006 Biblesoft, Inc. All rights reserved.)
Law is not of Faith
10 For all who rely on works of the law are under a curse; for it is written, “Cursed be everyone who does not abide by all things written in the Book of the Law, and do them.”
11 Now it is evident that no one is justified before God by the law, for “The righteous shall live by faith.”
12 But the law is not of faith, rather “The one who does them shall live by them.” ESV
Galatians 3:10-12 (KJV)
10 For as many as are of the works of the law are under the curse: for it is written, Cursed is every one that continueth not in all things which are written in the book of the law to do them.
11 But that no man is justified by the law in the sight of God, it is evident: for, The just shall live by faith.
12 And the law is not of faith: but, The man that doeth them shall live in them.
As we know “for whatsoever is not of faith is sin” (Romans 14:23) and in Galatians Paul tells us that the Law is not of faith. All through the OT we see that the just were saved by faith and not by the Law. The Law never saved any OT saint. The Law revealed our sin but Jesus fulfilled the Law and He saved us by faith in Him.
Are we justified through faith in Jesus or through the Law? If the Law is not of faith then we can’t be justified by faith and the Law.
So does it matter that Galatians was written by Paul? Did Paul change God’s instructions? Can we trust the words of Paul? Next we’ll look at Paul’s writings to see if we can trust them.