The Kingdom of Christ is fundamentally spiritual, but not merely spiritual
What makes a good king different from a bad king is the kind of power used and the way that power is used.
The thing that destines every king to fail is the unavoidable—and for every worldly king at some point irresistible—temptation to use their power to benefit themselves at the expense of others.
Devotional writer Jane Williams says,
“…what Jesus is offering as a description of his own kingship is truth—reality, you might say. Revelation calls it ‘the Alpha and Omega, who was and is and is to come’. If the actual reality of the world, from its creation to its end, is like Jesus, then this strange human obsession with power is an aberration. It has no ability to create, to redeem or to sanctify. Jesus’s challenge to Pilate’s kind of power is too slow and subtle for many of us, who long to use the weapons of worldly power to force victory for God. But if Jesus is the truth, then any other way is falsehood, and will fail. Reality, as it was and is and is to come, is shaped by a different kingship.”
Jesus lives as King over a kingdom that is certainly powerful, but he draws on a power not from this fallen world.
Jesus rules and reigns and fights battles in this world through the power of self-giving love and truth, which can only come from God himself.
His Kingdom is fundamentally spiritual in that it is conceived, birthed, and animated by the Spirit of Love who is God the Holy Spirit, but it is not merely spiritual because this Spirit takes up residence in his subjects, his followers, for the sake of the world.