(lib. de prop. sec. Deum.) Or, perhaps, the kingdom of God being within us, means that joy that is implanted in our hearts by the Holy Spirit. For that is, as it were, the image and pledge of the everlasting joy with which in the world to come the souls of the Saints rejoice.
Asked by the disciples when the kingdom of God would come, the Lord said, “The kingdom of God is within you,” through the truth of grace and not through the slavery of guilt. Let those that would be free be servants in the Lord. As we share in service, we also share in the kingdom. He said, “The kingdom of God is within you.” He would not say when it would come. He said there would be a day of judgment, so that he instilled in all terror of the judgment to come, and he did not add the guarantee of its postponement.
If the devil has been driven out and sin no longer reigns, then the kingdom of God is established in us. As it is written in the Gospel, “The kingdom of God does not come with observation, nor will they say, ‘Lo here,’ or ‘Lo, there.’ Truly I say to you that the kingdom of God is within you.” The only thing that can be “within us” is knowledge or ignorance of the truth and the affection for righteousness or sin by which we prepare our hearts to be a kingdom of Christ or the devil. St. Paul described the nature of this kingdom in this way: “For the kingdom of God is not food and drink, but righteousness and peace and joy in the Holy Spirit.” If the kingdom of God is within us and is righteousness, peace and joy, then someone that remains in these is surely within the kingdom of God. Someone that remains in unrighteousness, conflict and the melancholy that kills the life of the spirit is already a citizen of the devil’s kingdom, of hell and of death. These are the signs whether it is God’s kingdom or the devil’s.
Because our Saviour, in His discourses which He addressed to others, spake often of the kingdom of God, the Pharisees derided Him; hence it is said, And when he was asked by the Pharisees when the kingdom of God should come. As though they said tauntingly, "Before the kingdom of God come, which Thou speakest of, the death of the cross will be Thy lot." But our Lord testifying His patience, when reviled reviles not again, but the rather because they were evil, returns not a scornful answer; for it follows, He answered and said, The kingdom cometh not with observation; as if he says, "Seek not to know the time when the kingdom of heaven shall again be at hand. For that time can be observed neither by men nor angels, not as the time of the Incarnation which was proclaimed by the foretelling of Prophets and the heraldings of Angels." Wherefore He adds, Neither shall they say, Lo here! or, Lo there! Or else, They ask about the kingdom of God, because, as is said below, they thought that on our Lord's coming into Jerusalem, the kingdom of God would be immediately manifested. Therefore our Lord answers, that the kingdom of God will not come with observation.
Now it is only for the benefit of each individual that He says that which follows, For behold the kingdom of God is within you; that is, it rests with you and your own hearts to receive it. For every man who is justified by faith and the grace of God, and adorned with virtues, may obtain the kingdom of heaven.
These miserable men ask in mockery, “When will the kingdom of God come?” This is like saying, “Before this kingdom of which you speak comes, cross and death will seize you.” What does Christ reply? He again displays his long-suffering and incomparable love to humanity. Reviled, he does not revile again. Suffering, he does not threaten. He does not harshly scold them, but because of their wickedness, he does not stoop to give them an answer to their question. He says only what is for the benefit of all people: that the kingdom of God does not come by watching. “Behold, the kingdom of God is within you.” He says, “Do not ask about the times in which the season of the kingdom of heaven will again arise and come. Rather, be eager that you may be found worthy of it. It is within you. That is, it depends on your own wills and is in your own power, whether or not you receive it. Everyone that has attained to justification by means of faith in Christ and decorated by every virtue is counted worthy of the kingdom of heaven.”
“The kingdom of the heavens is within you.” You should not hope to find it in a place. It does not come in observation, according to the word of Christ.
" This means, "Neither in this place nor that place is the kingdom of God; for, behold, it is within you." And if the heretics, in their audacity, should contend that the Lord did not give an answer about His own kingdom, but only about the Creator's kingdom, concerning which they had inquired, then the following words are against them.
But they assert that not only is there in favour of their doctrine, testimony to be drawn from the mysteries of the Assyrians, but also from those of the Phrygians concerning the happy nature-concealed, and yet at the same time disclosed-of things that have been, and are coming into existence, and moreover will be,-(a happy nature) which, (the Naassene) says, is the kingdom of heaven to be sought for within a man. And concerning this (nature) they hand down an explicit passage, occurring in the Gospel inscribed according to Thomas, expressing themselves thus: "He who seeks me, will find me in children from seven years old; for there concealed, I shall in the fourteenth age be made manifest." This, however, is not (the teaching) of Christ, but of Hippocrates, who uses these words: "A child of seven years is half of a father." And so it is that these (heretics), placing the originative nature of the universe in causative seed, (and) having ascertained the (aphorism) of Hippocrates, that a child of seven years old is half of a father, say that in fourteen years, according to Thomas, he is manifested. This, with them, is the ineffable and mystical Logos. They assert, then, that the Egyptians, who after the Phrygians, it is established, are of greater antiquity than all mankind, and who confessedly were the first to proclaim to all the rest of men the rites and orgies of, at the same time, all the gods, as well as the species and energies (of things), have the sacred and august, and for those who are not initiated, unspeakable mysteries of Isis. These, however, are not anything else than what by her of the seven dresses and sable robe was sought and snatched away, namely, the pudendum of Osiris. And they say that Osiris is water. But the seven-robed nature, encircled and arrayed with seven mantles of ethereal texture-for so they call the planetary stars, allegorizing and denominating them ethereal robes,-is as it were the changeable generation, and is exhibited as the creature transformed by the ineffable and unportrayable, and inconceivable and figureless one. And this, (the Naassene) says, is what is declared in Scripture, "The just will fall seven times, and rise again." For these falls, he says, are the changes of the stars, moved by Him who puts all things in motion.
This, says he, was alone sufficient for its being understood by men; (I mean) the cup of Anacreon declaring, (albeit) mutely, an ineffable mystery. For dumb, says he, is Anacreon's cup; and (yet) Anacreon affirms that it speaks to himself, in language mute, as to what sort he must become-that is spiritual, not carnal-if he shall listen in silence to the concealed mystery. And this is the water in those fair nuptials which Jesus changing made into wine. This, he says, is the mighty and true beginning of miracles which Jesus performed in Cana of Galilee, and (thus) manifested the kingdom of heaven. This, says he, is the kingdom of heaven that reposes within us as a treasure, as leaven hid in the three measures of meal.
That the liberty of believing or of not believing is placed in free choice. In Deuteronomy: "Lo, I have set before thy face life and death, good and evil. Choose for thyself life, that thou mayest live." Also in Isaiah: "And if ye be willing, and hear me, ye shall eat the good of the land. But if ye be unwilling, and will not hear me, the sword shall consume you. For the mouth of the Lord hath spoken these things." Also in the Gospel according to Luke: "The kingdom of God is within you."
Nor will they say, "Look, here it is!" Because neither angels nor humans can observe when it will come, just as the time of the Lord's Incarnation, predestined by the most certain prophecies of the prophets, and declared by the proclamations of angels, so that his conception, birth, baptism, preaching, death, resurrection, and ascension to heaven were incessantly accompanied by signs from either angels or humans or certainly miracles.
Behold, the kingdom of God is within you. The Kingdom of God itself says it is placed within them, that is, in their hearts, where they believed it reigns. Hence it is written: The word is near you, in your mouth and in your heart, that is, the word of faith we preach (Rom. X).
[AD 395] Gregory of Nyssa on Luke 17:20-21