Essays on the Kingdom
Kingdom of God (or Christ): God's righteous reign in the earth, mediated by His Son, Jesus Christ. The kingdom of God begins in the hearts of redeemed men (Col. 1:13) and moves outward wherever men are subject to Christ's gospel and law. The kingdom of God is not preeminently political, though it has implications for politics. Wherever Christ's gospel breaks the stony heart of sinful man, bringing him to his knees in submission to Jesus Christ, there is the kingdom of God. As more and more men are converted and reorient their lives to the Bible, the kingdom of God extends throughout the earth in all spheres (vocation, technology, education, economics, science, the arts, and so forth).
P. Andrew Sandlin
Jesus' doctrine of the kingdom as both an inward and outward, coming first in the heart of man and afterwards in the external world, upholds the primacy of the spiritual and ethical over the physical. The invisible world of the inner religious life, the righteousness of the disposition, the sonship of God are in it made supreme, the essence of the kingdom, the ultimate realitites to which everything else is subordinate.
Geerhardus Vos (from The Teaching of Jesus Concerning the Kingdom of God and the Church)