Gal 5:4… you have fallen from grace. – This shows that you can receive God’s grace and then fall.
Quite true: but what kind of grace, and in what context? These words are addressed to a particular group, as the context shows:
Behold I, Paul, say to you that if you receive circumcision, Christ will be of no benefit to you. 3 And I testify again to every man who receives circumcision, that he is under obligation to keep the whole Law. 4 You have been severed from Christ, you who are seeking to be justified by law; you have fallen from grace.
So in what way had those seeking to be justified by law “fallen from grace”? The correspondent assumes, but does not prove, that to fall from grace proves that you once received saving grace; evidently, the same assumption would follow that one who is severed from Christ was once joined to Him savingly. Aside from the contradictions such a position creates with the plain assertions of Scripture elsewhere, the fact of the matter is Paul is addressing those who were seeking to add to faith in Christ the single act of obedience encompassed in circumcision–clearly the Judaizers were not saying you did not have to believe in Christ, nor were they importing the entirety of the law of Moses (Paul argues their inconsistency at this point as part of his refutation of them); instead, they were adding a select list of things one had to do in addition to faith to be right before God. Paul has already laid out the stark contrast between the path marked by law-keeping obedience and that marked by grace-inspired faith in Christ. One cannot go down both paths. These men were still seeking their justification, unlike true believers who look back upon theirs (Romans 5:1). They had not yet found peace with God by faith in Christ Jesus alone, and Paul says they will never find it going down the path they are going. They have been severed from Christ not in the sense that they had been salvifically united to Him and now He was failing to save them, but that by seeking to be made justified by something other than faith alone, they were severed from the only true source of life in Christ; they have fallen from grace not that they had been salvifically regenerated and justified and sanctified by grace already, and were now destroying that grace by their beliefs, but that they have fallen away or failed of grace (th/j ca,ritoj evxepe,sate) by proceeding down a path grace has never, and will never, mark out, that path of human cooperation and works righteousness that is so much the desire of the unregenerate heart.
To prove that God’s sovereign electing grace can fail to save the elect would require a text far more to the point than one addressing false teachers who are perverting the gospel and thus cutting themselves off from salvation.
