How do we know Christ's kingdom is a spiritual one?

Christ's kingdom is distinctly spiritual, characterized as one that dwells within the elect of God, as evidenced in the Gospel's proclamation. The spiritual nature is supported by verses such as Colossians 1:13, which speaks of being transferred into the kingdom of God's dear Son. This transfer involves a new birth, delivering believers from darkness and placing them in a state of grace governed not by external forces but by the inner workings of the Holy Spirit. This inward principle of holiness is reflected in the lives of believers, as they exhibit love, joy, and peace, indicating their true citizenship in an everlasting kingdom that is unshakable.