Paul tests his teaching against the most honored example his Jewish readers could name. Matthew Henry explains that "the Jews gloried much in their relation to Abraham," and counted being Abraham's seed among their chief privileges. Barnes calls the question "rather the objection of a Jew" and spells it out: "Did he derive no advantage in justification from the rite of circumcision, and from the covenant which God made with him?" Gill frames it the same way: did Abraham obtain these things "by his own strength? or were these acquired by his circumcision in the flesh"? If anyone could have earned a right standing with God, surely Abraham could.

Paul answers in two steps. First, if Abraham "were justified by works, he hath whereof to glory; but not before God" (Romans 4:2). Second, he goes to Scripture: "Abraham believed God, and it was counted unto him for righteousness" (Romans 4:3). So what Abraham "found" was real, but it was not a wage. Paul explains that to the worker the reward is "of debt," but to the one who believes, "his faith is counted for righteousness" (Romans 4:4-5).

The phrase "as pertaining to the flesh" can be read two ways, and Barnes lays out both. It may describe Abraham as their father by natural descent, which is how the ESV reads it ("our forefather according to the flesh") and how Gill first explains it: "according to carnal descent, or natural generation and relation." Or it may go with "hath found": what did Abraham gain by fleshly or outward things? Gill allows this too, saying the phrase "may be connected with the word found." Barnes calls this second reading "doubtless the proper connexion," and of the meanings offered for "flesh" he chooses circumcision: "This latter I take to be the correct interpretation."

The choice matters only a little. Either way the question asks whether anything outward, family line or ritual, made Abraham right with God. Barnes explains "hath found" simply as "Hath obtained."

This question shows that God's way of making people right with Himself has not changed. Paul does not invent a new path for Gentiles. He goes back to the founding father of Israel and shows that Abraham himself stood before God by believing, not by achieving. Not even Abraham had grounds to boast "before God" (Romans 4:2), which fits Paul's earlier words that boasting "is excluded" (Romans 3:27).

That makes Abraham "the father of us all" who believe (Romans 4:16), and it shows the kind of God he trusted: the One "that justifieth the ungodly" (Romans 4:5) and "quickeneth the dead" (Romans 4:17).