“For if Abraham was justified by works, he has something to boast about, but not before God. For what does the Scripture say? “Abraham believed God, and it was counted to him as righteousness.”” -Romans 4:2–3
In verse 2, Paul offers an enthymeme—an argument with an implied (because it should be obvious to all) premise—to set up his next train of arguments, that Abraham’s example proves righteousness is obtained by faith.
If the enthymeme in verse 2 were expanded into a complete syllogism, it could be understood to say: If Abraham was justified by works, he would have something to boast about (or glory in) before the throne of God. But he does not have any works to bring before God which would merit his own righteousness; therefore, he was not justified by works.
With this, Paul immediately moves to his real point to demonstrate, by testimony of the Scripture, that Abraham was justified by faith. To make the point, he quotes from Genesis 15:6: “And he [Abraham] believed the Lord, and he counted it to him as righteousness.”
To Abraham was imputed God’s righteousness because he simply believed what was promised him. Some contend that Paul stretches the use of this scenario to apply it to eternal salvation by faith in Christ. But that is not so. God’s promise to Abraham was a covenant of grace, like ours:
“And behold, the word of the Lord came to him: “This man shall not be your heir; your very own son shall be your heir.” And he brought him outside and said, “Look toward heaven, and number the stars, if you are able to number them.” Then he said to him, “So shall your offspring be.”” -Genesis 15:4–5
Whereby having taken possession of the covenant by faith, it was counted to Abraham as righteousness, which is the same thing as saying he was counted justified before the throne of God.
John Calvin commenting on this verse says, “Abraham, by believing, embraced nothing but the favour offered to him, being persuaded that it would not be void. Since this was imputed to him for righteousness, it follows, that he was not otherwise just, than as one trusting in God’s goodness, and venturing to hope for all things from him.”
Like Abraham, we today who are under the new covenant, are not justified by our own works. (If Abraham couldn’t be, we certainly couldn’t be.) Rather, the promise of the gospel (a covenant of grace) is brought to us—that is the good news of Christ’s work on our behalf—and we can only take possession of the promise by faith. If we believe God, it is counted to us as righteousness before the throne of God.