"You have become estranged from Christ, (...) you have fallen from grace." (Galatians 5:4).
Galatians 5:4 is one of the scariest verses in the Bible, at least to some people, because it is said that it proves a terrifying claim: Salvation can be lost. But you will notice, I didn't quote the whole verse. I quoted the first and last parts, but I had to remove the middle. This is because even if we take Galatians 5:4 alone, but do so in full, it gives away its true context.
When we read from verse 1 to verse 6, this is what we get: "Stand fast therefore in the liberty by which Christ has made us free, and do not be entangled again with a yoke of bondage. 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. For we through the Spirit eagerly wait for the hope of righteousness by faith. For in Christ Jesus neither circumcision nor uncircumcision avails anything, but faith working through love."
From this passage alone, we see the general theme of the book of Galatians. The entire epistle is a defence of the Biblical Gospel against a works-based fraud. A fraud so severe that, because the Galatians had so easily fallen for it, Paul does not even open his letter with his standard greeting. Watch how he opens most of his epistles:
Paul, a servant of Jesus Christ, called to be an apostle, separated unto the gospel of God, (...) To all that be in Rome, beloved of God, called to be saints: Grace to you and peace from God our Father, and the Lord Jesus Christ. - Romans 1:1, 7, emphasis added
Paul called to be an apostle of Jesus Christ through the will of God, and Sosthenes our brother, Unto the church of God which is at Corinth, to them that are sanctified in Christ Jesus, called to be saints, with all that in every place call upon the name of Jesus Christ our Lord, both their's and our's: - 1 Corinthians 1:1-2, emphasis added
Paul, an apostle of Jesus Christ by the will of God, and Timothy our brother, unto the church of God which is at Corinth, with all the saints which are in all Achaia: - 2 Corinthians 1:1, emphasis mine
Paul, an apostle of Jesus Christ by the will of God, to the saints which are at Ephesus, and to the faithful in Christ Jesus: - Ephesians 1:1, emphasis mine
Paul and Timotheus, the servants of Jesus Christ, to all the saints in Christ Jesus which are at Philippi, with the bishops and deacons: - Philippians 1:1, emphasis mine
Paul, an apostle of Jesus Christ by the will of God, and Timotheus our brother, To the saints and faithful brethren in Christ which are at Colosse: Grace be unto you, and peace, from God our Father and the Lord Jesus Christ. - Colossians 1:1-2, emphasis mine
Paul, and Silvanus, and Timotheus, unto the church of the Thessalonians which is in God the Father and in the Lord Jesus Christ: Grace be unto you, and peace, from God our Father, and the Lord Jesus Christ. - 1 Thessalonians 1:1, emphasis mine
Paul, and Silvanus, and Timotheus, unto the church of the Thessalonians in God our Father and the Lord Jesus Christ: - 2 Thessalonians 1:1, emphasis mine
Now read his greeting to the Galatians:
"Paul, an apostle, (not of men, neither by man, but by Jesus Christ, and God the Father, who raised him from the dead;) And all the brethren which are with me, unto the churches of Galatia: Grace be to you and peace from God the Father, and from our Lord Jesus Christ, Who gave himself for our sins, that he might deliver us from this present evil world, according to the will of God and our Father: To whom be glory for ever and ever. Amen." (Galatians 1:1-5).
In his greeting to the Galatians, Paul is brief and cold. He does not honor them as a "church in God our Father", or as saints and faithful brethren, or as those who are sanctified in Christ Jesus. He immediately proceeds, in verses 6-9, to say "I marvel that you are turning away so soon from Him who called you in the grace of Christ, to a different gospel, which is not another; but there are some who trouble you and want to pervert the gospel of Christ. But even if we, or an angel from heaven, preach any other gospel to you than what we have preached to you, let him be accursed. As we have said before, so now I say again, if anyone preaches any other gospel to you than what you have received, let him be accursed."
From this, we see the severity of Paul's charge. He is literally addressing a Gospel issue. In particular, the Galatians have fallen short of the Gospel itself. As a result, they haven't lost salvation - they have taken the wrong way to receive it in the first place.
The key word there is "receive". By Paul's reckoning, our salvation has very little to do with us. The actual effort is 100% on God's end. It is through the death and resurrection of Jesus Christ that salvation has been achieved, and this long before any of us were born. Even the ancients were saved by faith, though for them it was by faith in what was to come, rather than the cross that has now happened. To that end, Paul tells us "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." (Galatians 3:22-25).
Thus, ultimately, the entire epistle centers around one particular point: We are saved by grace, through faith, not of works (see Ephesians 2:8-10). So, to begin with, those who have "fallen from grace" are not losing salvation in any way traditionally assumed by believers in Conditional Security. They aren't losing their faith in Christ, they aren't failing to do good works, they aren't taking part in any particular evil. In truth, they are placing their faith in God, i.e. they believe in Him, and they are actively trying to obey His law, right down to the letter. They're getting circumcised, they're observing special times and seasons, they're separating themselves from Gentiles, and they're doing this to such an extent that even Peter requires Paul to straighten him out again.
But Paul says no, the law is a good thing, but it's insufficient to save us. Our works cost us our lives, because when we bind ourselves to it, we are bound to it, yet as he says in Romans 8:3-4, "For what the law could not do in that it was weak through the flesh, God did by sending His own Son in the likeness of sinful flesh, on account of sin: He condemned sin in the flesh, that the righteous requirement of the law might be fulfilled in us who do not walk according to the flesh but according to the Spirit." In other words, you have two choices: You can be saved by the grace of God, through faith in Christ, and let His works save you, or you can rely on your own works, which, by Isaiah's reckoning, are filthy rags (Isaiah 64:6).
So who are those who have "fallen from grace"? Are they those who have lost salvation? Paul makes it quite clear: There is one Gospel, and anyone who does not follow it is accursed. The Gospel is a faithful reliance upon God's grace. If you can lose that salvation, God is the one who failed! But these Galatians fell from grace because they jumped from grace, relying not on God's grace, but upon their own righteousness. They didn't lose salvation, they were trying to earn it. But because of our sin, none of us are able to do that.
With the context of Galatians 5:4 in mind, it's obvious that Paul is not denying Eternal Security here. If he was, no modern believer in Conditional Security could truly claim to have any kind of security! If security is based on faith, the Galatians had faith. If security is based on works, the Galatians had works. If the Galatians had faith and works, how did they lose salvation? Because the most essential element isn't there. Their faith was misplaced in their own works, meaning they never had salvation to lose. Thus, Paul was sent by God to redirect their attention away from the law, which was designed to point us to Christ.