Galatians 5:2-4
2 Behold, I Paul say unto you, that if ye be circumcised, Christ shall profit you nothing.
3 For I testify again to every man that is circumcised, that he is a debtor to do the whole law.
4 Christ is become of no effect unto you, whosoever of you are justified by the law; ye are fallen from grace.
While the above verse is actually part of an assault on works Salvation, the fact that if one is circumcised, Christ is said to "profit them nothing" (Galatians 2:2), and they are described as having "fallen from grace" (Galatians 2:4), has made this passage a prooftext for the idea that a person can lose their Salvation.
Does this passage teach that if we resort to Legalism, we lose our Salvation?
The Apostle Paul is clearly addressing believers, as the epistle is written to "brethren" (Galatians 1:11, 3:15, 4:12, 4:28, 4:31, 5:11, 5:13, 6:1, 6:18), with whom Paul frequently identifies with, speaking throughout in terms of "we", "our", and "us" (Galatians 1:3-4, 3:13, 3:23-26, 4:31, 5:1, 5:5, 6:9-10, etc.).
From this basis, we know that believers, the intended audience, have the New Birth (John 1:12, Galatians 3:26: 1 John 5:1), and that the New Birth is "incorruptible" (1 Peter 1:23), "cannot sin" (1 John 3:9, 5:18), and is already seated in Heaven with the Lord (Ephesians 2:6).
These facts, along with the myriad of other proofs that a Christian cannot lose their Salvation, point us to look toward an experiential sense of the words "profit" and "fall", as they are used in the passage in question.
The immediate context of the verse in question contrasts the liberty afforded to us by Christ, with the "yoke of bondage" of the Law:
Galatians 5:1
1 Stand fast therefore in the liberty wherewith Christ hath made us free, and be not entangled again with the yoke of bondage.
The Law is referred to as "bondage" throughout the epistle (Galatians 2:4, 4:9, 4:24-25, 5:1), and also a "curse" (Galatians 3:10) that we have been redeemed from (Galatians 3:13).
Elsewhere in Scripture, this Law of bondage is contrasted with "grace" explicitly:
Romans 6:14
14 For sin shall not have dominion over you: for ye are not under the law, but under grace.
So, what we are seeing is that the Law of bondage is not something we are under anymore, in the New Testament. Instead, the entire Law has been replaced with just two Great Commandments from the old Law (Matthew 22:37-40).
This new Law is variously called "the Law of liberty" (James 1:25, 2:12), "the Law of Christ" (Galatians 6:2), "the Law of the Spirit of life in Christ Jesus" (Romans 8:2), and the "royal Law" (James 2:8). Just like the old Law, it's not a basis for justification (Romans 3:20, Galatians 3:11, etc.) - especially seeing that it constitutes a subsection of the old Law (Deuteronomy 6:5, Leviticus 19:18). It also is not the basis for an eternal condemnation, unlike the old Law, which was exactly that, for all of us, before we believed (Romans 3:19-20, Galatians 3:19).
In the passage in question, elsewhere in Scripture (Acts 15:1-4), and especially throughout this epistle (Galatians 2:4-14, 3:1-3, 4:9-11, etc.), there are warnings against those who would try to bring us back to the Law of bondage:
Galatians 2:4
4 And that because of false brethren unawares brought in, who came in privily to spy out our liberty which we have in Christ Jesus, that they might bring us into bondage:
For those who would leave the Law of liberty, and regress to the Law of bondage - which includes everything commanded by God in the Old Testament - they can be said to have "fallen from grace" (Galatians 5:4), and that Christ is "profiting them nothing" (Galatians 5:2), because they have now become a "debtor to do the whole Law" (Galatians 5:3, 3:10, James 2:10). In their experience, they have been "entangled with the yoke of bondage" (Galatians 5:1) that Christ died to redeem them from.
This means, in essence, they are living as if Christ died for no reason (Galatians 2:21). They have fallen from the grace afforded them by the Law of liberty, that we have as believers in Christ, which is done by nature by those walking in the Spirit (Romans 8:4, Galatians 5:16-18, etc.).
Part of the "better Testament" (Hebrews 7:22) is a Law established on "better promises" (Hebrews 8:6). Unlike in the Old Testament, where there were arduous laws governing every aspect of life, we have been placed under a Law whose points can be kept, unconsciously, simply by yielding to the Spirit.
This is the grace that we've been given in the New Covenant, as one of our benefits for being in Christ. If someone were to eschew that, and try to return to the Law of bondage that Christ fulfilled, they would be doing despite to grace, and the payment of Christ. That's what the passage in question is warning against. It has nothing to do with invalidating someone's Salvation, it just ensures that they won't enjoy it, here in their experience.