Israel's failure needed explaining. They had pursued righteousness, while the Gentiles had not, and yet the Gentiles received it and Israel did not (Romans 9:30-31). Paul's one-word question, "Wherefore?", asks the reason. The problem was not a lack of effort but the way they sought it. Barnes puts it this way: "They depended on their own righteousness, and not on the mercy of God to be obtained by faith."

Paul answers at once: "Because they sought it not by faith, but as it were by the works of the law. For they stumbled at that stumblingstone" (Romans 9:32). He backs this up from Scripture: "Behold, I lay in Sion a stumblingstone and rock of offence: and whosoever believeth on him shall not be ashamed" (Romans 9:33).

"Wherefore" in Greek is dia ti, literally "because of what?" Paul's quotation in Romans 9:33 draws on Isaiah 28:16 and Isaiah 8:14, which Barnes notes are combined here. In Isaiah, God Himself would be "a stone of stumbling and for a rock of offence to both the houses of Israel" (Isaiah 8:14). The Textus Receptus (the Greek text behind the KJV) reads "works of the law"; many later Greek editions read only "works," which is why the ESV says "as if it were based on works." Matthew Henry (Concise) explains that they "expected justification by observing the precepts and ceremonies of the law of Moses."

The same Christ who is a sure foundation for those who believe is a stumblingstone for those who try to reach God by their own works. Gill says they sought righteousness in the law, not in Christ, "in whom only are righteousness and strength." Matthew Henry (full commentary) writes that "Christ himself is to some a stone of stumbling."

Yet the verse that follows ends in hope: "whosoever believeth on him shall not be ashamed" (Romans 9:33). And Paul's very next words are a prayer "that they might be saved" (Romans 10:1). The stone that trips the self-reliant is the stone that holds up everyone who trusts it.