“Keep close to the word preached. The word preached, is virga virtutis, the rod of God’s strength; it is the great engine he uses for setting up the kingdom of grace in the heart. ‘Faith cometh by hearing’ (Rom 10.17). Though God could work grace immediately by his Spirit, or by the ministry of angels from heaven, yet he chooses to work by the word preached. This is the usual mean, by which he sets up the kingdom of grace in the heart; and the reason is, because he has put his divine sanction upon it; he has appointed it for the means of working grace, and he will honor his own ordinance (1 Cor. 1.21)…. Let us keep the word preached, because the power of God goes along with it.”
Watson also describes the kingdom of God in a twofold way. The kingdom of grace is God’s kingdom on earth, which culminates in the kingdom of glory, which is “that glorious estate which the saints shall enjoy when they shall reign with God and angels for ever” (heaven itself). “These two kingdoms of grace and glory, differ not specifically, but gradually; they differ not in nature, but in degree only. The kingdom of grace is glory in the seed, and the kingdom of glory is grace in the flower. The kingdom of grace is glory militant, and the kingdom of glory is grace triumphant…. The kingdom of grace leads to the kingdom of glory.”
The whole section on “Your Kingdom Come” in Watson’s book, The Lord’s Prayer (Carlisle: Banner of Truth, 1982), 54-150, is amazingly helpful and detailed. If you want a good summary of the Kingdom of God from a Reformed/Presbyterian perspective, read this section of Watson.
Note: “The kingdom of grace” and “the kingdom of glory” are pretty common terms in older Reformed theology. For example, Wilhelmus ‘A Brakel (around the same time as Watson) uses the same terms in The Christian’s Reasonable Service (under the Lord’s Prayer).