Ambrosiaster was an anonymous Latin Christian writer of the late fourth century, active most likely in Rome during the pontificate of Pope Damasus I. Though once mistaken for Ambrose of Milan, he is now distinguished from him and known by the scholarly name “Ambrosiaster,” meaning “Pseudo-Ambrose.” Little is known about his personal life, but his surviving commentary on Paul’s epistles became an important witness to early Western biblical interpretation. Writing with careful attention to the text of Scripture, Ambrosiaster frequently emphasizes the priority of God’s grace, the insufficiency of human works for justification, and the centrality of faith in Christ. For this reason, he remains an especially significant voice for understanding how Pauline theology was read in the Latin Church before Augustine and the later medieval tradition. 

“But to the one who does not work”—that is, to the one who is subject to sins, who does not do what the law commands—“but believes in Him who justifies the ungodly, his faith is reckoned as righteousness.” He says this because, apart from the works of the law, the faith of the ungodly person—that is, the Gentile—who believes in Christ is reckoned as righteousness, just as Abraham’s was.

How, then, do the Jews think that they are justified by the works of the law with the justification of Abraham, when they see that Abraham was not justified by the works of the law, but by faith alone? Therefore the law is not needed when the ungodly person is justified before God by faith alone.

He says that this was according to the purpose of God’s grace: that, with the law having ceased, the grace of God would require faith alone for salvation. 

Just as David also says. He confirms this very thing by the example of the prophet: the blessedness of the man to whom God credits righteousness apart from works. He says that those are blessed concerning whom God has decreed this: that apart from labor and any observance, they are justified before God by faith alone.”

Ambrosiaster, Comm. in Rom. 4:5–6, PL 17:48  [bold added]

“Where then is your boasting? It is excluded. By what law? Of works? No, but by the law of faith. Having given his reason, he speaks to those who live under the Law, saying that they have no reason to boast upon themselves concerning the Law and claiming to be of the race of Abraham, seeing that no one is justified before God except through faith.”

Ambrosiaster, Comm. in Rom. 3:27, PL 17:47. [bold added]