Going back to the law means that they have been severed from Messiah.
The question here is not grace and law, but grace or law.The Greek word translated as severed or ” fallen away” is ‘katargeo’, which means ” to render inoperative.”
He is not saying that they lose their salvation. What Paul is saying is that seeking either justification or sanctification by means of the law will render their relationship to the Messiah inoperative.This is the same Greek word that is used in Romans 7:2 and 6 in reference to the Mosaic Law; the law has been rendered inoperative. They will not be able to rely on the Spirit to keep the righteousness of God. Instead, they are fallen away from grace. The word simply means “to fall short.” It is used the same way in Hebrews 12:15. They will not lose their salvation, but they will deprive themselves of the ministry of the Spirit.The term “falling from grace” means that they will fall from the power given to them for daily living because one can either work in the sphere of the law or in the sphere of the Spirit; it is not both law and Spirit.
Taken from Faith AloneThe Condition of Our Salvation An exposition of the Book of Galatians. By Arnold G. Fruchtenbaum
Question: “What does it mean to ‘fall from grace’ (Galatians 5:4)?”
Answer: Galatians 5:4 and its reference to falling from grace is one of those “warning passages” pointed to by those who reject the doctrine of the eternal security of the believer. But because of the biblical doctrine of the perseverance of the saints, we know the warnings cannot be directed at true believers in Christ because once grace has been obtained, believers cannot fall from it.
In Galatians 5:4, the context is Paul’s warning against mixing law and the Gospel to attain justification. He says to those who let themselves be circumcised (Galatians 5:2) that they are “trying to be justified by law” and have therefore “been alienated from Christ; you have fallen away from grace.” It should be noted that there is no mention of salvation or the security of the believer. He is telling those who receive circumcision—in other words attempt to justify themselves through the rites and rules of the Law—that Christ will be of “no benefit” to them.
Paul expounds further in verse 3 when he says that “every man who receives circumcision” is “under obligation to keep the whole Law.” Why is such a statement important in regards to Christ being “no benefit to you”? Note what Paul says in Galatians 3:13 concerning Christ’s sacrifice: “Christ redeemed us from the curse of the Law, having become a curse for us.” Taken in this light, along with a brief understanding of the Greek terms used, we can get a better understanding of what Paul is saying.
The two most important words in Galatians 5:4 are καταργέω (severed) and ἐκπίπτω (fallen). The word καταργέω does not require the “harsh” implications that come with “sever” in the English language, though it should not be taken lightly. In light of Paul’s words in Galatians 5:2 and how Paul uses the term in verses such as Romans 3:3, 4:14, and 1 Corinthians 1:28, a good way of understanding the term καταργέω is “nullify” or “done away with.” While the word ἐκπίπτω can undoubtedly mean to fall away from a previously held position, as those who deny the eternal security of true believers assert, in the context of this verse and how it is used in passages such as Acts 27, a good understanding of the phrase in Galatians 5:4 is that of “estranged” or “separated from.”
Paul warns against setting aside the grace that comes from Christ. Those who do have nullified, or run away from, the grace that comes through His blood and attempted instead to justify themselves by the works of the Law. The purpose of Paul’s letter to the Galatians was to warn against the Judaizers because they attempted to lure born-again Christians back to justification through the Law, which is impossible (Galatians 2:16). He reminds them of the freedom they have in Christ: “It is for freedom that Christ has set us free. Stand firm, then, and do not let yourselves be burdened again by a yoke of slavery”
(Galatians 5:1).https://www.gotquestions.org/fall-from-grace.html