"That is why it depends on faith, in order that the promise may rest on grace and be guaranteed to all his offspring."
Grace and the works of sinners are at odds in justification. The Lord's legal declaration that we are righteous in His sight comes through either grace alone or our works alone. There can be no mixing of the two. This truth is summarized in the Reformation slogan sola gratia—grace alone. Saving grace is necessary, sufficient, and efficient for justification. It ensures that we will believe and be declared righteous, so it guarantees God's promise to all of Abraham's offspring, to those who rest only on God's promises (Rom. 4:16a).
Our keeping of the law cannot be involved in this because, as we saw yesterday, the law of God can pronounce only wrath upon the sinner. Grace is not entirely absent from the Mosaic law, broadly considered. After all, the sacrifices to atone for sin were God's gracious provision for His sinful people. But when Paul speaks of the law bringing only wrath in Romans 4:13–15, he is not thinking of every rule written in the five books of Moses but rather the law as God's standard for righteousness. All that this standard can do is affirm us when we keep it flawlessly and judge us when we fail. The law as God's righteous standard brings wrath because it identifies transgressions. Without law, there is no transgression (v. 15). This does not mean there is no sin without the Lord's revealed law, as Paul explains in 5:13. Instead, what the revealed standard of righteousness does is turn sin as a general phenomenon into specific transgressions of divinely revealed commandments. The more light one has, the more heinous his rejection of that light (Luke 12:35–48).
Every descendant of Adam is "under sin" apart from Jesus (Rom. 3:9–20). Putting any hope at all in the law to justify us—even if a little bit and even if we say grace helps us keep the law to complete our justification—nullifies the promise and keeps us from being its rightful heirs (4:14). Even when empowered by grace, we cannot keep the law perfectly, and God demands perfection for our justification (2:13). Thus, justification must be by grace alone. Once we try to mix our imperfect obedience with the Lord's acts, we fall short of His perfection. God alone can save, so His grace and our works are opposed in justification.
Justification by faith alone preserves the principle of grace alone. Saving faith as depicted in Scripture is an empty hand that receives only what God has done to redeem His people. It does not bring good works as if to ask the Lord to declare us righteous because of these deeds. This kind of faith is the only kind of faith that preserves justification by divine grace.
Coram Deo
Dr. R.C. Sproul comments in his commentary on Romans: "It is Christ's righteousness that justifies us. All we bring to the table is our trust in Him and His righteousness. If we add one ounce of our own righteousness as our confidence, we repudiate the gospel." Those of faith will do good works, but they do not bring those works to God and ask Him to justify them based upon their deeds. Faith alone means grace alone, and grace alone means that God alone gets the glory for our salvation.