Galatians 5:
1It was for freedom that Christ set us free; therefore keep standing firm and do not be subject again to a yoke of slavery. 2Behold I, Paul, say to you that if you receive circumcision, Christ will be of no benefit to you.3And I testify again to every man who receives circumcision, that he is under obligation to keep the whole Law. 4You have been severed from Christ, you who are seeking to be justified by law; you have fallen from grace
Self-explanatory.
"Severed from Christ" implies attachment, proving true salvation. Sinners were never attached to Christ to begin with.
"Fallen from grace" implies you had to be there first. There can be no other meaning but this.
Here's the problem. You're focusing solely on the words "fallen from grace" and allowing them to confirm the context, while disregarding the rest of it. It's like when catholics pick all the scriptures that say "fire" and then try to make a case for "purgatory".
The bit I highlighted in red makes the context clear. "You who are seeking to be justified by law" - these are people who aren't saved, but are trying to be saved by the law. So then how can they lose something they don't yet have?
So then "you've fallen from grace" in light of the correct context is saying they have rejected grace and embraced law to save them. Just like if someone were to give me their opinion a lot, I might tell them they've fallen away from scripture.
There's a connection to other Galatians scriptures.
In Galatians 3 there's mention of baby believers trying to be sanctified by the flesh. They're trying to be made perfect by the flesh, so have become entangled in that yoke of legalism. Here in Galatians 5 it mentions non believers who are trying to be be justified by the flesh. So you have some believers trying to be sanctified by the flesh and some non believers trying to be justified by the flesh.
Verse 1-3 addresses believers - warns not to fall into the trap of being sanctified by the flesh (legalism)
Verse 4 addresses non believers - those who are trying to be justified by the flesh
Verse 5-6 addresses believers
Galatians 5:1-6: 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 (because/since we) (believers)
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"