How do we know God's kingdom is a spiritual kingdom?

In the sermon, it is articulated that many Jews during Paul's time were looking for a physical kingdom, yet Paul preached the reality of a spiritual kingdom—the kingdom of God—which is present among believers now. This spiritual kingdom is based on grace, truth, and the righteousness of Christ, rather than political or earthly power. The church, as part of this kingdom, is composed of those who have been called out by God and transformed by His Spirit. Therefore, our citizenship is in heaven, emphasizing that we are part of a divine kingdom ruled by Christ himself, the true King.