In a wonderful and creative way, Paul summarizes his doctrine of justification by faith alone in a positive way in verse 20, and then in the (often overlooked) next verse he warns us sternly about the consequences of neglecting or perverting the free grace of Christ. Luther's comments on the passage are almost as powerful as the passage itself.
The good news
Paul writes, "I have been crucified with Christ; and it is no longer I who live, but Christ lives in me; and the life which I now live in the flesh I live by faith in the Son of God, who loved me and gave Himself up for me."
Luther sums up the basic meaning of the verse with his usual personal, practical, and pastoral concern:
Paul explains what constitutes true Christian righteousness. True Christian righteousness is the righteousness of Christ who lives in us. We must look away from our own person. Christ and my conscience must become one, so that I can see nothing else but Christ crucified and raised from the dead for me. If I keep on looking at myself, I am gone. If we lose sight of Christ and begin to consider our past, we simply go to pieces. We must turn our eyes to the brazen serpent, Christ crucified, and believe with all our heart that He is our righteousness and our life. For Christ, on whom our eyes are fixed, in whom we live, who lives in us, is Lord over Law, sin, death, and all evil....
Since Christ is now living in me, He abolishes the Law, condemns sin, and destroys death in me. These foes vanish in His presence. Christ abiding in me drives out every evil. This union with Christ delivers me from the demands of the Law, and separates me from my sinful self. As long as I abide in Christ, nothing can hurt me. Christ domiciling in me, the old Adam has to stay outside and remain subject to the Law. Think what grace, righteousness, life, peace, and salvation there is in me, thanks to that inseparable conjunction between Christ and me through faith!
When I got to the last part of the verse in Luther's commentary, I knew he would probably wax eloquent on the wonderful words there about Christ's love and sacrifice. I looked forward to seeing how he would "speak in thunderbolts," as Spurgeon said about him. But little did I know that Luther himself would describe Paul's words in that way...
The words, "The Son of God who loved me, and gave Himself for me," are so many thunderclaps and lightning bolts of protest from heaven against the righteousness of the Law. The wickedness, error, darkness, ignorance in my mind and my will were so great, that it was quite impossible for me to be saved by any other means than by the inestimable price of Christ's death.
And then Luther throws out a few of his own thunderbolts when he comments on the words "for me"...
Who is this "me"? I, wretched and damnable sinner, dearly beloved of the Son of God. If I could by work or merit love the Son of God and come to Him, why should He have sacrificed Himself for me?....Read the words "me" and "for me" with great emphasis. Print this "me" with capital letters in your heart, and do not ever doubt that you belong to the number of those who are meant by this "me." Christ did not only love Peter and Paul. The same love He felt for them He feels for us. If we cannot deny that we are sinners, we cannot deny that Christ died for our sins.
The bad news
The passage doesn't end there, however, as Paul goes on to say in verse 21, "I do not nullify the grace of God, for if righteousness comes through the Law, then Christ died needlessly.” Here is an implicit warning that becomes much more explicit later in the book (3:4, 4:11, 5:2-4): if we add anything to faith in Christ alone, or trust in anything else in addition to Him, we will not be saved. (And remember, he's talking to very religious people who call themselves Christians.)
On that topic, Luther has a few more thunderbolts to hit us with...
We despise the grace of God when we observe the Law for the purpose of being justified. The Law is good, holy, and profitable, but it does not justify. To keep the Law in order to be justified means to reject grace, to deny Christ, to despise His sacrifice, and to be lost.
If my salvation was so difficult to accomplish that it necessitated the death of Christ, then all my works, all the righteousness of the Law, are good for nothing. How can I buy for a penny what cost a million dollars? The Law is a penny's worth when you compare it with Christ. Should I be so stupid as to reject the righteousness of Christ which cost me nothing, and slave like a fool to achieve the righteousness of the Law?