Why would he need to give this instruction if they were never saved?
Fall from grace in the context of those verses mean, went from the true gospel of grace unto a fallen gospel with the law added which ruins the whole thing.... this is less about who is and is not saved on a personal level to the church, and more about providing sound doctrine and describing how dangerous this gospel which doesn't rely on the righteousness of Christ which is imputed by grace through faith...
No?
Galatians 5
5 Stand fast therefore in the liberty wherewith Christ hath made us free, and be not entangled again with the yoke of bondage.
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.
5 For we through the Spirit wait for the hope of righteousness by faith.
6 For in Jesus Christ neither circumcision availeth any thing, nor uncircumcision; but faith which worketh by love.
7 Ye did run well; who did hinder you that ye should not obey the truth?
8 This persuasion cometh not of him that calleth you.
9 A little leaven leaveneth the whole lump.
10 I have confidence in you through the Lord, that ye will be none otherwise minded: but he that troubleth you shall bear his judgment, whosoever he be.
11 And I, brethren, if I yet preach circumcision, why do I yet suffer persecution? then is the offence of the cross ceased.
12 I would they were even cut off which trouble you.
13 For, brethren, ye have been called unto liberty; only use not liberty for an occasion to the flesh, but by love serve one another.
14 For all the law is fulfilled in one word, even in this; Thou shalt love thy neighbour as thyself.
15 But if ye bite and devour one another, take heed that ye be not consumed one of another.
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This is to the whole church which would have a wide group of different people in different spots.....
Though paul is confident in those that actually believe the truth through the LORD to keep us, but understands there are those among us who are dangerous and will push for the specific false doctrine of following laws to be justified... then instructs us to walk in love
22 But the fruit of the Spirit is love, joy, peace, longsuffering, gentleness, goodness, faith,
23 Meekness, temperance: against such there is no law.
Lead by example instead if tit for tat.... those who look to the law or works are the quickest to point their fingers lol
We ought to show the peace and joy that comes from the good news...
(though I admit I find it hard when I see someone pervert what is so important to me)
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TLDR: I believe "fallen from grace" here is descriptive of separate principals where one is to obey the truth and submit to the righteousness of Christ which is by GRACE through faith and the opposite where circumcision or any law will play even a small part in how we are seen as righteous before God.... which would turn grace into something else
This is a message to the church which includes saved and unsaved which all believe they are "in Christ" but paul remains confident that through the Lord all who are His will ultimatelty not be other minded than what is true even if our walk is hindered and we have arguments so fruitless that it would have been better to just cut off what it is that caused these fights... ( given circumcision is the topic... goodness😬)
I believe the "instruction" is to walk in love... which is to believers