Paul said if the saved Galatians become slaves of a gospel of works for justification they will not be heirs of the Holy Spirit and the promise of glorification. We can follow his argument in the letter he wrote to them and see that he is telling them they will not receive the inheritance if they stop believing in Christ for justification:
The Galatians are turning away from the gospel of Christ for justification and turning toward a false gospel of justification through works of the law:
6 "I am amazed that you are so quickly deserting Him who called you by the grace of Christ, for a different gospel; 7which is really not another"-Galatians 2:6
He tells them even we Jews know that a person can't be justified by works of the law and can only be justified through faith in Christ:
16 "nevertheless knowing that a man is not justified by the works of the Law but through faith in Christ Jesus, even we (Jews) have believed in Christ Jesus, so that we may be justified by faith in Christ and not by the works of the Law"-Galatians 2:16
Here we see that the Galatians did in fact get saved and received the Spirit. He's talking to saved people who are turning away from justification by faith in Christ to justification through works of the law. This is NOT about people who were never saved:
3 "Are you so foolish? Having begun by the Spirit, are you now being perfected by the flesh (trying to earn justification through perfect law keeping)?"-Galatians 3:3
He points out that the blessing given to Abraham was given through a promise (not law), and that the promise is fulfilled in the giving of the Holy Spirit:
13 "Christ redeemed us from the curse of the Law, having become a curse for us... 14 ...in order that in Christ Jesus the blessing of Abraham might come to the Gentiles, so that we would receive the promise of the Spirit through faith."-Galatians 3:13-14
Paul equates the promised blessing, the Holy Spirit, to the inheritance. The promised inheritance, given through faith in Christ is the Holy Spirit/salvation/eternal life.
18 "For if the inheritance is based on law, it is no longer based on a promise; but God has granted it to Abraham by means of a promise."-Galatians 3:18
He reaffirms that it is those who belong to Christ who are heirs of the promise, the Holy Spirit/salvation:
27For all of you who were baptized into Christ have clothed yourselves with Christ.
29And if you belong to Christ, then you are Abraham’s descendants, heirs according to promise.
They are known to God in salvation, but want to be slaves to justification through the works of the law:
9But now that you have come to know God, or rather to be known by God, how is it that you turn back again to the weak and worthless elemental things, to which you desire to be enslaved all over again? 10You observe days and months and seasons and years."-Galatians 4:9-10
Paul tells them slaves of the law are not heirs. Even the law they want to be under says that:
21Tell me, you who want to be under law, do you not listen to the law?
30But what does the Scripture say?
“CAST OUT THE BONDWOMAN AND HER SON,
FOR THE SON OF THE BONDWOMAN SHALL NOT BE AN HEIR WITH THE SON OF THE FREE WOMAN.”
Once again he reaffirms their status as saved, free children of the New Covenant and exhorts them to resist the slavery of justification by the law. Which will cause them to no longer be heirs of the promises made to Abraham (the Holy Spirit/salvation/eternal life):
31 "So then, brethren, we are not children of a bondwoman, but of the free woman.
1 It was for freedom that Christ set us free; therefore keep standing firm and do not be subject again to a yoke of slavery."-Galatians 4:31,5:1
If they rely on the works of the law for justification (epitomized in circumcision) they lose the efficacy of Christ in justification:
2 "Behold I, Paul, say to you that if you receive circumcision, Christ will be of no benefit to you."-Galatians 5:2
The person who seeks to be justified by works of the law becomes severed from Christ and from the grace of justification through faith:
4 "You have been severed from Christ, you who are seeking to be justified by law; you have fallen from grace."-Galatians 5:4
The hope of glorification comes through the Spirit, by faith, but as he has told them they lose the promised inheritance of the Spirit if they become slaves to the law for justification:
5 "For we through the Spirit, by faith, are waiting for the hope of righteousness."-Galatians 5:5
There it is, folks. You lose the grace of justification in Christ, and the Holy Spirit in salvation/eternal life if you fall away from faith in Christ. It's as plain as day for all of us to read if you'll read it with open eyes and an honest and open heart and not through the prejudice of a 'once saved always saved' doctrine.
The Galatians are turning away from the gospel of Christ for justification and turning toward a false gospel of justification through works of the law:
6 "I am amazed that you are so quickly deserting Him who called you by the grace of Christ, for a different gospel; 7which is really not another"-Galatians 2:6
He tells them even we Jews know that a person can't be justified by works of the law and can only be justified through faith in Christ:
16 "nevertheless knowing that a man is not justified by the works of the Law but through faith in Christ Jesus, even we (Jews) have believed in Christ Jesus, so that we may be justified by faith in Christ and not by the works of the Law"-Galatians 2:16
Here we see that the Galatians did in fact get saved and received the Spirit. He's talking to saved people who are turning away from justification by faith in Christ to justification through works of the law. This is NOT about people who were never saved:
3 "Are you so foolish? Having begun by the Spirit, are you now being perfected by the flesh (trying to earn justification through perfect law keeping)?"-Galatians 3:3
He points out that the blessing given to Abraham was given through a promise (not law), and that the promise is fulfilled in the giving of the Holy Spirit:
13 "Christ redeemed us from the curse of the Law, having become a curse for us... 14 ...in order that in Christ Jesus the blessing of Abraham might come to the Gentiles, so that we would receive the promise of the Spirit through faith."-Galatians 3:13-14
Paul equates the promised blessing, the Holy Spirit, to the inheritance. The promised inheritance, given through faith in Christ is the Holy Spirit/salvation/eternal life.
18 "For if the inheritance is based on law, it is no longer based on a promise; but God has granted it to Abraham by means of a promise."-Galatians 3:18
He reaffirms that it is those who belong to Christ who are heirs of the promise, the Holy Spirit/salvation:
27For all of you who were baptized into Christ have clothed yourselves with Christ.
29And if you belong to Christ, then you are Abraham’s descendants, heirs according to promise.
They are known to God in salvation, but want to be slaves to justification through the works of the law:
9But now that you have come to know God, or rather to be known by God, how is it that you turn back again to the weak and worthless elemental things, to which you desire to be enslaved all over again? 10You observe days and months and seasons and years."-Galatians 4:9-10
Paul tells them slaves of the law are not heirs. Even the law they want to be under says that:
21Tell me, you who want to be under law, do you not listen to the law?
30But what does the Scripture say?
“CAST OUT THE BONDWOMAN AND HER SON,
FOR THE SON OF THE BONDWOMAN SHALL NOT BE AN HEIR WITH THE SON OF THE FREE WOMAN.”
Once again he reaffirms their status as saved, free children of the New Covenant and exhorts them to resist the slavery of justification by the law. Which will cause them to no longer be heirs of the promises made to Abraham (the Holy Spirit/salvation/eternal life):
31 "So then, brethren, we are not children of a bondwoman, but of the free woman.
1 It was for freedom that Christ set us free; therefore keep standing firm and do not be subject again to a yoke of slavery."-Galatians 4:31,5:1
If they rely on the works of the law for justification (epitomized in circumcision) they lose the efficacy of Christ in justification:
2 "Behold I, Paul, say to you that if you receive circumcision, Christ will be of no benefit to you."-Galatians 5:2
The person who seeks to be justified by works of the law becomes severed from Christ and from the grace of justification through faith:
4 "You have been severed from Christ, you who are seeking to be justified by law; you have fallen from grace."-Galatians 5:4
The hope of glorification comes through the Spirit, by faith, but as he has told them they lose the promised inheritance of the Spirit if they become slaves to the law for justification:
5 "For we through the Spirit, by faith, are waiting for the hope of righteousness."-Galatians 5:5
There it is, folks. You lose the grace of justification in Christ, and the Holy Spirit in salvation/eternal life if you fall away from faith in Christ. It's as plain as day for all of us to read if you'll read it with open eyes and an honest and open heart and not through the prejudice of a 'once saved always saved' doctrine.