"Fallen from grace" carries many connotations today. Here is its originally intended meaning: To trust anything but the grace of God to save you. "For by grace you have been saved, through faith, [and [even] that [faith (or the grace) is] not of yourself." It is simply a gift. Grace is grace and that is all it is. Add something to it and you have it no more.

If the fare has been paid, and you throw in your two dollars, it's not a free trip any longer. It is an insult to the one who offered to pay it all.

Notice it is "to attempt" to be justified by law. “You are seeking to be justified by law.” KJV has the literal correctness here, implying that some Galatians were actually being justified by law. You cannot really be justified this way. Though we must say that the Greek only supports the idea of justified, we understand why modern translators have supplied the "attempt" or “seeking” idea, so as not to mislead.

Paul says they are "justified in law", but in the same verse says they are fallen from grace and estranged from Christ, hence not really justified at all.

So what does “fallen from grace” mean here? Loss of salvation? Fallen from being saved, since grace is what saves? Luther takes all this very seriously:

“The words ‘Ye are fallen from grace’ must not be taken lightly. They are important. To fall from grace means to lose the atonement, the forgiveness of sins, the righteousness, liberty, and life which Jesus has merited for us by His death and resurrection. To lose the grace of God means to gain the wrath and judgment of God, death, the bondage of the devil, and everlasting condemnation.”

If grace picks you up, then grace must let you go if you reach out to the Law. When grace lets go, you fall.

In the Luther context, there were Galatians who had heard the message of grace, and delighted in it at first, but never subscribed to it before those wicked Judaizers came a long like the birds in Jesus’ parable of the four soils, snatched the word out of their hearts, and caused them to trust in Moses.

“Severed from Christ” in the first part of that verse is no less severe than “fallen from grace” in the last part. Cut off from Jesus just as the good seed was making its way into their hearts, as they were distracted from the truth by a lie. Paul’s warning is serious here.

Can I repent before it is too late, before I “fall”?

Others, like Ironside, are not as clear on this point. He says, “Falling from grace… is turning from the full, clear, high Christian standard of salvation by grace alone, to the low level of attempting to keep one’s salvation by human effort.” In his view, we are to repent of such thinking the moment we are made aware of it, but not that we have lost out with God forever.

He tells the story of a man who asked a Methodist minister, “I understand that you Methodists believe in falling from grace; is that so?”

The Methodist minister answered, “I understand that you Presbyterians believe in horse-stealing.”

“We do not!”

“Well, don’t you believe it’s possible for a man to steal a horse?”

“Yes, but we wouldn’t do it.”

“Well, we believe it is possible for men to fall from grace, but we do not believe in doing it.”