Q. Why does Jesus seem to focus so much on “the kingdom of God” while Paul seems to focus on “salvation by grace”? Are they two sides of the same thing, or are they different focuses?
I think it is fair to say that Jesus’ teaching was essentially about the kingdom of God. That’s how Matthew, Mark, and Luke actually summarize his teachings in their gospels—in Luke, for example: “Jesus traveled about from one town and village to another, proclaiming the good news of the kingdom of God.”
But I don’t think it’s quite accurate to say that Paul’s teaching is essentially about salvation by grace. As Gordon Fee has shown quite convincingly in his book God’s Empowering Presence (abridged in Paul, the Spirit, and the People of God), we actually have that impression about Paul because we read him through the eyes of the Reformation.
The Reformers were trying to respond to a situation in which salvation was being depicted, explicitly or implicitly, as the result of works. Beginning with Luther, the Reformers found in the opening parts of epistles such as Galatians and Romans a strong insistence that salvation is by grace alone, through faith alone, and they made this the centerpiece of their response. All Protestants are heirs of the Reformation and so we see Paul through this lens.
But Paul was not actually writing to counter people who taught salvation by works. Rather, he was correcting the teaching that salvation was by grace and sanctification was then by works—specifically works such as circumcision (the main issue in Galatians) or keeping kosher and Jewish festivals (as in Colossians).
So Paul emphasizes salvation by grace, not works, in the earlier parts of such epistles so that he can make the argument that sanctification should and must come by similarly by God’s work in us, not our work for God. As he challenges the Galatians, “Are you so foolish? Having begun by the Spirit, are you now being perfected by the flesh?”
This reference to the Spirit is the key element. Paul’s opponents said that people, once saved by grace, needed to be trained and restrained by the law; otherwise, what would keep them from running wild? Paul countered that the transforming influence of the Holy Spirit in believers’ lives would change them into people who desired and delighted to obey God.
But the very fact that the Holy Spirit was and operating in this fashion was, for Paul, evidence that the “age to come” or the “kingdom of God” was breaking into human history. In other words, the essential incentive to live by the Spirit and not by the flesh is that we are already citizens and heirs of the kingdom of God.
Towards the end of Romans, for example, Paul turns aside a legalistic concern for sanctification through keeping kosher in favor of the transforming influence of the Spirit by explaining, “The kingdom of God is not a matter of eating and drinking but of righteousness and peace and joy in the Holy Spirit.” And he warns the Galatians against the “deeds of the flesh” by reminding them that “those who do such things will not inherit the kingdom of God”—in other words, those who will inherit the kingdom of God do not do such things.
So we see that the notion of the “kingdom of God” is not an emphasis that’s central to Jesus’ teaching but missing from Paul’s. Rather, for Paul the presence of the kingdom is evidenced in the coming of the Spirit and in the Spirit’s transforming influence, which allows people not only to be saved by grace rather than works, but also to be sanctified by the Spirit rather than by works.