How do we know the kingdom of God is spiritual?

The nature of God's kingdom is fundamentally spiritual, as outlined in Romans 14:17, where it is identified with righteousness, peace, and joy rather than external rituals or legalistic practices. The apostle Paul emphasizes that the kingdom does not come with outward show or rules but is rather within us, a transformative reality of grace. This contrasts with the Old Covenant, where there were strict laws and rituals. In the New Covenant, Christ's work establishes His kingdom in the hearts of believers through the Holy Spirit, thereby fulfilling God's promises.