John F. MacArthur's Exposition on Galatians 5:4

An excerpt from Pastor John F. Macarthur's message "The Security of Salvation Part 2" (which was taken from Pastor Brett's blog) writes:
Galatians 5:4…Galatians 5:4. And it says there, "Christ is become of no effect unto you whosoever of you are justified by the law, ye are fallen from grace." And people say – You see, right there it proves that you can fall from grace. 
That’s right, it does say that. But would you notice to whom it says that?  It says it to people who try to get saved by…what?…by law. You go back to verse 2: "Behold, I Paul say unto you that if you be circumcised," in other words, if you believe you can be saved by surgery, if you believe you can be right with God by some kind of physical operation, "then Christ means nothing to you." Christ is of no use to you. You don’t need Him. He’s profitless because you can be saved by your surgery. 
And then in verse 4 he says: "Christ is also of no effect to you if you are justified by the law." In other words, if you think you can be made right with God by your law-keeping and your self- righteousness and your own religious works, then equally Christ is useless to you and these are the kinds of people who are fallen from grace. What does it mean? It means that you have fallen away from the grace principle of salvation. It really isn't defining Christians in terms of salvation. 
It’s defining non-Christians…people who come to God, as it were, or attempt to come to God some other way than through grace. You are fallen, as it were, away from the true principle, the true principle that saves is grace, it is not teaching us that a Christian standing in grace can fall out of grace. The context would be utterly foreign to that concept. We who are saved through the Spirit, verse 5, we wait for the hope of righteousness by faith, not by law. So it is simply falling from the principle of grace as a way of salvation. 
Now, Christ’s death, then, provides for us these two things, peace with God and standing in grace. Now I want to show you the third link and I don’t know if we’ll get pass this, I intended to do two each time, but this is so rich. The third link, verse 2 again, "We have access by faith into the grace in which we stand, and we rejoice," or we exult, or actually we boast, we make our boast, "in hope of the glory of God.” The third link in our security is hope of glory. 
We are secure because we have peace with God. We are secure because we stand in grace. And we are secure because we have been given the hope of glory. In other words, to put it another way, God has promised us future glory, right? He promised. Does God keep His promises?