Indeed I, Paul, say to you that if you become circumcised, Christ will profit you nothing. And I testify again to every man who becomes circumcised that he is a debtor to keep the whole law. You have become estranged from Christ, you who attempt to be justified by law; you have fallen from grace. Galatians 5:2-4

Paul wrote chastising the Galatians for teaching and believing that law-keeping was necessary for salvation. The specific law that was at issue was Jewish circumcision. The legalists insisted that male Gentile believers needed to be circumcised to become Jews in addition to trusting in Christ to become Christians. Paul used this specific error to teach the broader error of insisting that keeping any law for salvation was as wrong as wrong could possibly be. He made two essential points:

First, if a person makes circumcision, or any other aspect of the law, necessary for salvation, keeping the entire law would then be necessary: “he is a debtor to keep the whole law” (v.3). (Refer to the previous blog post for a more detailed explanation of that point.)

Second, those who insist on law-keeping as necessary for salvation “have become estranged from Christ,” and “have fallen from grace.” What does this mean, and not mean?

Paul uses shocking words to drive the importance of his point home. Associating law-keeping with salvation is contrary to the gospel and is therefore an exceeding grave error.
Those who teach such things remove themselves from Christianity, distancing themselves from Christ and the gospel.
Does this mean that a true Christian can lose his salvation for believing and teaching such things? True Christians can be in serious error (the Galatians are a case in point), but if they are true Christians, their error cannot undo true salvation.
However, if they are true Christians, they will repent. If they do not repent, they only reveal that they were never really saved. (See 1 John 2:19.)
And, by the way, it doesn’t matter if one is making law-keeping the sole means of salvation, or is adding law-keeping to faith in Christ. Both are equally wrong because salvation is received through faith in Christ, alone!