Peter Van Mastricht (d. 1709 – a Reformed professor of theology at Frankfort, Duisburgh, and Utrecht Universities) wrote a theological work called Theoretico-Practica Theologia in 1699. Soli Deo Gloria published part of it in a booklet called A Treatise on Regeneration. The entire book is well worth reading, but for now I just want to point out how he, as with other Reformed scholastics of his day, talked about the Kingdom(s) of God.
“The use or purpose to which regeneration is subservient: entering into the kingdom of God. The kingdom of God here signifies both the kingdom of grace upon the earth, or the church (Matt 4.17, Col 4.11) …and also the kingdom of glory in heaven (Luke 14.15, 23.42), with all things which pertain to both these kingdoms (Luke 8.10), i.e., all spiritual blessings” (p. 7).
As I’ve noted before, this “speak” is the standard “speak” of the old-school Reformers (English to German to Dutch). It is completely safe to say that Mastricht agreed with and used the typical kingdom(s) distinction: The kingdom of power, the kingdom of grace, and the kingdom of glory. The kingdom of power has to do with God the Creator ruling over all things by his sovereign providence in a non-redeeming way. The kingdom of grace/glory has to do with God the Redeemer’s saints on earth (kingdom of grace) and the glorified elect in heaven (kingdom of glory). Bavinck also uses these terms: “On earth He [Christ] rules as king, by His grace in the church, by His power in the world…” (Our Reasonable Faith, 559; cf. 382).
This also has to do with the reign of Christ: regnum potentiae (reign/kingdom of power), regnum gratiae (reign/kingdom of grace – having to do with the church militant), and the regnum gloriae (reign/kingdom of glory – having to do with the church triumphant). There is one King, but he rules differently over different realms. See Muller’s Latin/Greek dictionary for more on those scholastic distinctions.
I’m thinking that the scholastics used “kingdom” in a similar way that they used “covenant.” That is, while they taught the covenant of works, they spoke primarily of “the covenant” with reference to the covenant of grace. So with the kingdom: while they taught the kingdom of power, they spoke primarily of “the kingdom” with reference to the kingdom of grace/glory. The above quote by Mastricht, I’d submit, is proof – he wanted to remind the reader of these distinctions in his discussion (cf. William Ames’ comments on Lord’s Day 48).