Yesterday we spoke of how Sihon, king of the Amorites, who is the lofty one and the barren tree, having engaged with Israel was defeated; concerning whom Scripture also says pointedly that he fell "by the slaughter of the sword" or, as we read elsewhere, "by the mouth of the sword." But if you wish to know more carefully by what sword this barren and lofty one, "like the cedar of Lebanon," fell, let us learn from the Apostle Paul, who says: "For the word of God is living and effective and more piercing than any sword," and again in another place: "and the sword," he says, "of the spirit, which is the word of God." By the slaughter of this sword, then, fell the spiritual Sihon, who is the devil. After these things it is added: "and the sons of Israel," he says, "had dominion over all his land." All this earthly region indeed is called the land of Sihon, but Christ and his Church have dominion over all the land of Sihon. "And they had dominion from Arnon as far as Jabbok." Arnon and Jabbok were cities of the kingdom of Sihon, but the beginning of his kingdom was Arnon and the end was Jabbok, and therefore it is said that "they had dominion from Arnon as far as Jabbok." Now Arnon is interpreted "their curses"; the beginning, therefore, of the kingdom of this lofty and barren Sihon is curses. But the end is Jabbok, which is interpreted "wrestling"; for it is necessary that everyone who wishes to go out from the kingdom of the devil and to escape his borders should find wrestling, and that contests be stirred up against him by his ministers and satellites. But if he has wrestled and conquered, then Jabbok will no longer be a city of Sihon, but will be a city of Israel. This is doubtless what we read also concerning the patriarch Jacob, since, when he had come to a certain place, a wrestling was prepared for him, and there, when he had wrestled the whole night and had prevailed and grown strong before God, his name was called Israel. "And Israel took," he says, "all these cities; and Israel dwelt in all the cities of the Amorites." This Israel, who is Israel in Christ, who is Israel not in the flesh, nor a Jew in what is manifest, he himself dwells in all the cities of the Amorites when in all the world the churches of Christ are being propagated. But each one of us also was formerly a city of King Sihon, the lofty king; for there reigned in us folly, pride, impiety, and all the things that are on the devil's side; but when the strong one was overcome and vanquished and his vessels plundered, we were made cities of Israel and the inheritance of the saints—if indeed that power which formerly ruled over us has been utterly cut off in us, and the barren tree has been cut down, and the lofty king has been cast down, and we are under that king who says: "learn from me, for I am meek and humble of heart." After these things he now enumerates one by one the cities of the proud and barren king, and says: "in Heshbon and in all its neighboring places." What is this, that he named Heshbon as the chief city of the kingdom of Sihon? Heshbon is interpreted "thoughts." Rightly, then, does the greatest part of the diabolical kingdom and the greater part of its power reign in thoughts; for so the Lord also said: "out of the heart proceed evil thoughts, murders, adulteries, thefts, false testimonies, blasphemies; and these are the things that defile a man." And therefore this city is necessarily set ablaze and consumed by fire. By what fire? Surely by that of which the Savior says: "I came to send fire upon the earth, and how I wish that it be kindled!"
Source: HOMILIES ON NUMBERS 13.1