"Then Isaiah is so bold as to say, 'I have been found by those who did not seek me; I have shown myself to those who did not ask for me.' But of Israel he says, 'All day long I have held out my hands to a disobedient & contrary people.'"
Paul, who grew up a strict & devout Jew, is writing about his longing for his own people to come to embrace Jesus. He still believes they are God's "chosen people," but recognizes the mission has also expanded to the Gentiles (something that devout Jews find incredulous). Nevertheless, he has a firm hope that God will still redeem His people, though they've strayed.
The fact that God's mission has expanded is not surprising to Paul. He cites Isaiah who wrote that God would be found "by those who did not seek me." I love thinking about that in today's context. God isn't spending all His time hanging out in churches, waiting for people to come to Him. He's actively engaged in the lives of people all over the world - reaching out to them, even when they haven't (yet) expressed any interest in God!
Along that same vein, Paul cites another passage from Isaiah... where God laments that He's been holding out his open hands to many of us who call ourselves "believers," yet we've ignored or dismissed him. Why? It's an age-old question. One that is couched in our sinful humanity. Forgive us for ignoring you, Lord. Forgive me. Open me up to embrace you fully. Your outstretched arms are wide enough for so many! Let me take nothing for granted.