Romans 9:32,33

‘For what reason? Because (they sought) not by faith, but as it were by works. They stumbled at the stone of stumbling, even as it is written, “Behold, I lay in Zion a stone of stumbling and a rock of offence, and he who believes on him will not be put to shame.”

And why did they fail to ‘arrive at the Law'? That is fail to fulfil it to the Law's satisfaction. It was because they had sought to fulfil it in the wrong way. They had thought that they could achieve it ‘by works', that is, by hard endeavour, and by their own efforts. And many had struggled manfully to that end, like Paul had once done, but they had inevitably failed, because for sinful man it was unachievable. Thus what they should rather have done was respond to the righteousness of God which was by faith in their Messiah, in Jesus Christ (Romans 3:22), receiving it as a free gift (Romans 3:24; Romans 5:15). Then the righteousness of the Law would have been fulfilled in them (Romans 8:4).

But to believe in Jesus Christ Who had brought them the true significance of the Law (Matthew 5-7), and Who had brought righteousness through faith in Him (Romans 5:14), was beyond them. For if He was right then they, and all they had lived for, were wrong. They stumbled at (the verb contains the idea of responding in annoyance to) the stumblingstone of which the Scriptures had spoken, the stumblingstone of the Messiah. (As men always stumble at and are annoyed with God's ways). He was a stumblingstone because the way of salvation that He had brought was contrary to the ideas of men, and in their eyes, with their false emphasis, was contrary to the Law of Moses. Christ crucified was for them a stumblingblock (1 Corinthians 1:23). They had failed to see that the Law of Moses and the prophets pointed to a righteousness of God obtainable through Christ and through His death (Romans 3:21; Romans 3:24; Leviticus 1-16; Isaiah 53:11). And so their pride in their own viewpoint was too great to enable them to accept His offer. They were so tied up with religious forms and ceremonies, and with the ‘traditions of the elders', and were so proud of them, that as a result His way appeared too simple. It offended their religious perspectives and attitudes. And so He became both a stumblingstone, a stone which tripped them up, and a rock of offence, a rock on which they hurt themselves.

Paul then illustrates this with citations from Scripture which had by this time come to be seen by many as referring to the Messiah (this reference of it to the Messiah is found e.g. in some of the Targums, the Aramaic paraphrases of the Old Testament Scriptures which had been developed for synagogue use). His citation is “Behold, I lay in Zion a stone of stumbling and a rock of offence, and he who believes on him will not be put to shame.” This is a combination of Isaiah 28:16 with Isaiah 8:14; Isaiah 28:16 reads, ‘ Behold I lay in Zion  for a foundation,  a stone, a tried stone, a precious cornerstone, of sure foundation.  He who believes  will not make haste (LXX  will not be put to shame).' Isaiah 8:14 reads, ‘and He will be for a sanctuary, but for  a stone of stumbling and for  a rock of offence  to both the houses of Israel.' Paul thus conflates the two verses (which was, as we have previously seen, a general method of the day) in order to bring out that for the majority of Israel hope and sureness were replaced by unbelief and stumbling. He takes the opening and closing clauses in Isaiah 28:16 and inserts within them a portion (paraphrased) of Isaiah 8:14 because, sadly, He Who was intended for a foundation and a Sanctuary for Israel, was to turn out rather to be a stumblingstone and rock of offence for a large part of Israel. On the other hand, for those who believed in Him there would be nothing to be ashamed of. They could rest confidently in Him without shame, not racing about trying to find a solution. Thus he sees the unbelief of a large part of Israel concerning the Messiah as already prophesied in Scripture.

Interestingly this same combination of citations is found in 1 Peter 2:6 (although not conflated, and including another ‘stone' quotation) suggesting that it was well recognised in the early church that these verses referred to Christ. Paul will cite Isaiah 28:16 LXX again in Romans 10:11.