1 We acknowledge, beloved brethren, that we are owing you, and ought now to repay, what was left over for consideration, how we can understand that there is no real mutual contrariety between these two statements, namely, that after saying, “In my Father's house are many mansions: if it were not so, I would have told you, that I go to prepare a place for you;”— where He makes it clear enough that He said so to them for the very reason that there are many mansions there already, and there is no need of preparing any; — the Lord again says: “And if I go and prepare a place for you, I will come again and receive you unto myself; that where I am, there ye may be also.” How is it that He goes and prepares a place, if there are many mansions already? If there were not such, He would have said, “I go to prepare.” Or if the place has still to be prepared, would He not then also properly have said, “I go to prepare”? Are these mansions in existence already, and yet needing still to be prepared? For if they were not in existence, He would have said, “I go to prepare.” And yet, because their present state of existence is such as still to stand in need of preparation, He does not go to prepare them in the same sense as they already exist; but if He go and prepare them as they shall be hereafter, He will come again and receive His own to Himself: that where He is, there they may be also. How then are there mansions in the Father's house, and these not different ones but the same, which already exist in a sense in which they can admit of no preparation, and yet do not exist, inasmuch as they are still to be prepared? How are we to think of this, but in the same way as the prophet, who also declares of God, that He has [already] made that which is yet to be. For he says not, Who will make what is yet to be, but, “Who has made what is yet to be.” Therefore He has both made such things and is yet to make them. For they have not been made at all if He has not made them; nor will they ever be if He make them not Himself. He has made them therefore in the way of fore-ordaining them; He has yet to make them in the way of actual elaboration. Just as the Gospel plainly intimates when He chose His disciples, that is to say, at the time of His calling them; and yet the apostle says, “He chose us before the foundation of the world,” to wit, by predestination, not by actual calling. “And whom He did predestinate, them He also called;” He has chosen by predestination before the foundation of the world, He chooses by calling before its close. And so also has He prepared those mansions, and is still preparing them and He who has already made the things which are yet to be, is now preparing, not different ones, but the very mansions He has already prepared: what He has prepared in predestination, He is preparing by actual working. Already, therefore, they are, as respects predestination; if it were not so, He would have said, I will go and prepare, that is, I will predestinate. But because they are not yet in a state of practical preparedness, He says, “And if I go and prepare a place for you, I will come again, and receive you unto myself.”
2. But He is in a certain sense preparing the dwellings by preparing for them the dwellers. As, for instance, when He said, “In my Father's house are many dwellings,” what else can we suppose the house of God to mean but the temple of God? And what that is, ask the apostle, and he will reply, “For the temple of God is holy, which [temple] you are.” This is also the kingdom of God, which the Son is yet to deliver up to the Father; and hence the same apostle says, “Christ, the beginning, and then they that are Christ's in His presence; then [comes] the end, when He shall have delivered up the kingdom to God, even the Father;” that is, those whom He has redeemed by His blood, He shall then have delivered up to stand before His Father's face. This is that kingdom of heaven whereof it is said, “The kingdom of heaven is likened unto a man who sowed good seed in his field. But the good seed are the children of the kingdom;” and although now they are mingled with tares, at the end the King Himself shall send forth His angels, “and they shall gather out of His kingdom all things that offend. Then shall the righteous shine forth as the sun in the kingdom of their Father.” The kingdom will shine forth in the kingdom when [those that are] the kingdom shall have reached the kingdom; just as we now pray when we say, “Your kingdom come.” Even now, therefore, already is the kingdom called, but only as yet being called together. For if it were not now called, it could not be then said, “They shall gather out of His kingdom everything that offends.” But the realm is not yet reigning. Accordingly it is already so far the kingdom, that when all offenses shall have been gathered out of it, it shall then attain to sovereignty, so as to possess not merely the name of a kingdom, but also the power of government. For it is to this kingdom, standing then at the right hand, that it shall be said in the end, “Come, you blessed of my Father, receive the kingdom;” that is, you who were the kingdom, but without the power to rule, come and reign; that what you formerly were only in hope, you may now have the power to be in reality. This house of God, therefore, this temple of God, this kingdom of God and kingdom of heaven, is as yet in the process of building, of construction, of preparation, of assembling. In it there will be mansions, even as the Lord is now preparing them; in it there are such already, even as the Lord has already ordained them.
Source: Tractates on the Gospel of John (New Advent)