3 But why is it that He went away to make such preparation, when, as it is certainly we ourselves that are the subjects in need of preparation, His doing so will be hindered by leaving us behind? I explain it, Lord, as I can: it was surely this You signified by the preparation of those mansions, that the just ought to live by faith. For he who is sojourning at a distance from the Lord has need to be living by faith, because by this we are prepared for beholding His countenance. For “blessed are the pure in heart, for they shall see God;” and “He purifies their hearts by faith.” The former we find in the Gospel, the latter in the Acts of the Apostles.
But the faith by which those who are yet to see God have their hearts purified, while sojourning at a distance here, believes what it does not see; for if there is sight, there is no longer faith. Merit is accumulating now to the believer, and then the reward is paid into the hand of the beholder. Let the Lord then go and prepare us a place; let Him go, that He may not be seen; and let Him remain concealed, that faith may be exercised. For then is the place preparing, if it is by faith we are living.
Let the believing in that place be desired, that the place desired may itself be possessed; the longing of love is the preparation of the mansion. Prepare thus, Lord, what You are preparing; for You are preparing us for Yourself, and Yourself for us, inasmuch as You are preparing a place both for Yourself in us, and for us in You. For You have said, “Abide in me, and I in you.” As far as each one has been a partaker of You, some less, some more, such will be the diversity of rewards in proportion to the diversity of merits; such will be the multitude of mansions to suit the inequalities among their inmates; but all of them, none the less, eternally living, and endlessly blessed.
Why is it that You go away? Why is it You come again? If I understand You aright, Thou withdrawest not Yourself either from the place You go from, or from the place You come from: You go away by becoming invisible, You come by again becoming manifest to our eyes. But unless You remain to direct us how we may still be advancing in goodness of life, how will the place be prepared where we shall be able to dwell in the fullness of joy? Let what we have said suffice on the words which have been read from the Gospel as far as “I will come again, and receive you to myself.”
But the meaning of what follows, “That where I am, there ye may be also; and whither I go ye know, and the way ye know,” we shall be in a better condition— after the question put by the disciple, that follows, and which we also may be putting, as it were, through him— for hearing, and more suitably situated for making the subject of our discourse.
Source: Tractates on the Gospel of John (New Advent)