According to Ambrosiaster, St. Paul speaks here as though saying: “I cannot prove ungrateful to the grace of God, nor fail to abide unshaken in faith in the Lord Jesus Christ. I know that no one has benefited me as He has. If I compare Him with another, namely the law, from which I received no help, I thereby show contempt for Him.”
“For if righteousness comes through the law,” δικαιοσυνη, that is, righteousness, a God-pleasing and saving life, “then Christ died in vain.” If the law bestowed both forgiveness of sins and inward uprightness and sanctification, then there was no reason for Christ to die.
He died in order to grant us these two essential blessings: forgiveness of sins and sanctifying grace. Apart from Him no one else could give them, and without them we have no salvation.
As long as we are under sin and unforgiven, we have no access to God. Even if we were forgiven, yet not inwardly renewed, and did not receive strength to walk unwaveringly in the will of God, we would still remain unfit before God.
The Lord and Savior died for us and nailed our sins to the Cross. Then, having ascended into heaven, He sent down the Holy Spirit from the Father. Therefore He grants those who believe in Him both forgiveness of sins and the saving grace of the Holy Spirit.
Without these two there is no salvation for us. Consequently, Christ, who obtained them for us, did not die in vain. But if the law bestowed these blessings, there would have been no reason for Christ to die, since the law would already provide everything needed for salvation.
The Apostle, however, states here only the general proposition. He leaves each person to draw the conclusion from it, because it is utterly evident. Namely: but Christ did not die in vain. It cannot possibly be said that Christ died needlessly. Therefore righteousness is not by the law.
St. Chrysostom says concerning these words of St. Paul: “What sin could be greater than this? What words could be stronger for reproof? If Christ died, it is clear that the law had no power to justify us. If the law justifies, then Christ died in vain.
But would it not be senseless to call useless and vain this work, so fearful and surpassing human understanding? This is the ineffable mystery which the patriarchs awaited with longing, which the prophets foretold, and upon which the angels looked with awe.
This work must rightly be acknowledged by all as the chief cause of God’s providence for us. Thus the Apostle presents it as the height of folly and absurdity to regard so great and weighty a work as vain.”
“This word is very strong,” writes Blessed Jerome, “against those who, after faith in Christ, think that the prescriptions of the law must be observed. If righteousness comes through the law, then Christ died in vain.
Or let them show how Christ did not die in vain, if the works of the law justify. Yet however foolish they may be, they will not dare to say that Christ died for nothing.
Therefore we must complete the argument here by the following inference: but Christ did not die in vain. We must conclude: therefore righteousness is not through the law.”