Paul is concerned here with explaining the place of Israel in the purposes of God and here the apostle writes about whether or not Israel at that time had heard the gospel, how they responded to it, and whether there were any benefits accompanying their response.
The apostle points out in verse 15 that the gospel had been sent to the Israelites and uses a verse from Isaiah 52 to illustrate how the gospel came to them. The message of the cross was first declared to Israelites and we could think about how beautiful the feet of the messengers were on the Day of Pentecost as the gospel was declared to those who had rejected Jesus as Messiah a few weeks before that day. Those who responded in faith discovered that they had been delivered by Jesus from bondage far more severe than the captivity in Babylon to which that verse from Isaiah 52 had referred originally. They had been delivered from the penalty and power of sin, had been forgiven and reconciled to God, when they had called upon him.
But in verses 16 and 17, Paul comments sadly that not all of them responded correctly to the gospel. The apostle moves his eye slightly down the scroll of Isaiah and mentions the first verse of Isaiah 53, a verse in which Isaiah in his day lamented the fact that no one was paying heed to his prophecies about the coming Messiah. It is significant that Paul connects the gospel message to that chapter in Isaiah because it refers to the sufferings of Jesus as well as to his glory. And that helps us understand what Paul means by ‘the word of Christ’ in verse 17.
Paul asks in verse 18 whether or not the Israelites had heard the message and answers his own question by quoting from Psalm 19:4, a verse that refers to how the creation speaks of God all the time and in every place. We should not imagine that Paul is choosing a verse at random, but see instead that he is indicating that in a way similar to how God speaks universally in creation so he speaks to people about the gospel, a reminder of the rapid progress that the gospel can make. After all, the people of Israel had been scattered throughout the world for centuries and often Christian churches came from the synagogues after they gospel had been preached there.
The fact that the Israelites have heard the gospel leads Paul to make three more comments about them and the coming into God’s kingdom of the Gentiles (vv. 19-21). First, he points out that God had judged the Israelites because they had rejected the message after understanding its contents (v. 19). The judgement was that the Jews would become jealous and angry when God opened the door of his kingdom to Gentiles. Is this not what happened frequently in the Book of Acts? Paul is informing his readers that they should not be surprised at the hostile conduct of the Jews because Moses had predicted in Deuteronomy 32:21 when he stated that was how God would treat the Israelites when they rejected his message. So Paul says that the Israelites in general are under divine judgement because of their rejection of the gospel.
Second, he reminds his readers that the refusal of Israel did not mean that the kingdom would be without members (v. 20). Instead Paul cites from Isaiah 65:1 and says that it predicted that the Gentiles, who previously had no interest in God, would become his people. All Paul needed to do was get his readers to look around them and see that the kingdom of God was extending before their eyes, even although the Israelites had rejected the message.
Third, Paul stresses that the judgement on Israel and the ingathering of Gentiles did not mean that God did not want to bless Israelites. Rather the Lord was imploring them continually to return to him (v. 21). Here we see that God desires to show mercy even to those under his judgement. In the next chapter Paul will explain how mercy will even be shown to disobedient Israel. But for the present we should consider the amazing fact that the Lord desires to bless those whose conduct brings his judgement upon them.