Of how great importance the doctrine of the kingdom of God is, may be recognized even from this: that the heavenly Master, Christ, began His sermons with these words: Do penance, for the kingdom of heaven is at hand (Matt. 4:17). And He framed nearly all His parables about the kingdom of God, saying: The kingdom of heaven is likened to… (Matt. 13; 18; 20; 21; 22; 25) And after His resurrection, during those last forty days before the Ascension, appearing to His disciples, He spoke of the kingdom of God, as St. Luke testifies in the Acts of the Apostles. Thus the beginning, the progress, and the consummation of Christ's preaching was the kingdom of God (Acts 1). We, however, do not attempt in this place to explain all that can be discussed about the kingdom of God, but only those things which pertain to the place and state of the saints; and first of all, why the place and state of the saints is called the kingdom of God in the Sacred Writings.
The dwelling place of the saints is called the kingdom of God for many reasons. First, because it is a most vast region, far vaster than the narrow bounds of human thought can grasp. The globe of the earth — which is but a point, if compared with the highest heaven — contains many great kingdoms, so that they can scarcely be numbered: how great, then, will be that kingdom which is one alone, and is spread abroad through the whole breadth and expanse of the heaven of heavens? Nor does the kingdom of God contain only the region above the heavens, but this whole universe of things. For the region above the heavens, which is properly called the kingdom of God, is, as it were, the first province of the kingdom of God, in which the chief princes reside, who are all sons of God. The second province may be called the ethereal region, in which the stars dwell; and these stars, although they are not in themselves living beings, nevertheless so obey the nod of the Creator that they may be called living and animate, according to the words of the Church: Come, let us adore the King for whom all things live. The third province is the region of air, in which the winds and clouds run their course, and produce storms, rains, snows, hail, thunder, and lightning, and in which also birds of diverse kinds move about and fly. The fourth province is the watery region, which embraces the seas, springs, rivers, and lakes, in which are bred the fish that pass through the paths of the sea (Ps. 8). The fifth is the earthly region, which, as though emulating heaven, has most noble inhabitants, though not most blessed — namely, men endowed with reason, but mortal — who nevertheless have dominion over the beasts of the earth, the birds of the air, and the fish of the sea. The last is the subterranean province, which, like a desert Arabia, produces no good fruits at all, but only thorns and thistles; in it dwell the reprobate spirits, who by their pride merited this: that they who wished to be first should become the last, and that they who strove to exalt their throne above the stars of heaven should be thrust down to the lowest hell. And this place awaits all those men who, as companions of the demons, wallowed in shameful deeds and crimes, and depart from this life without true repentance. All these provinces God holds under His dominion, to whom the Psalmist sings: All things serve You (Ps. 118). And this whole most vast and greatest kingdom God will share with those who love Him, as we shall say a little further on.
Therefore, Christian soul, enlarge your heart; do not let yourself be hemmed in by the narrow confines of present things. Why do you toil and sweat so much to obtain a tiny fragment of this world, when you could possess the whole of it, if you wished? Surely if mortals aspired to this kingdom in earnest, or even thought of it attentively, they would blush to wage wars for the tiniest little portions of earth. God offers you, O man, a share in His own immense and eternal kingdom; and you, for the defense or acquisition of a single little town, wage war, in which many robberies and murders and innumerable other sins are committed, by which the King of kings is most justly provoked to anger. Where is your prudence? Where is your counsel? Nor do I say this because I think it unlawful for Christians to wage war in defense of their towns: for I know that just wars are defended not only by the holy Fathers, especially Augustine, a most learned and most holy man (Epistle 5 to Marcellinus), and by St. Thomas, the prince of scholastic theologians (II-II, q. 40); but that soldiers were also told by the very Precursor of the Lord, than whom no one greater has arisen among those born of women, not to abandon the military life as though it were unlawful, but, content with their pay, to do violence to no one (Luke 3:14) and to wrong no one; and I myself defended just wars in my books of the Controversies (Book III, On the Laity, ch. 14). I do not, then, simply condemn wars, but exhort to what is more perfect and often more profitable; in much the same way as St. Paul says to the Corinthians: It is a fault in you that you have lawsuits among yourselves; why do you not rather take wrong? Why do you not rather suffer yourselves to be defrauded? (1 Cor. 6:7) And St. James in his Epistle: From whence are wars and strifes among you? Are they not from your concupiscences? You covet, and have not; you kill and envy, and cannot obtain; you quarrel and wage war, and you have not (James 4:1–2). For surely a man who was deeply concerned about the kingdom of God would not readily be stirred to war over the loss of a single little town, but would seek mediators who would settle the dispute without the expense and the perils of wars. But let us pass on to the rest.