6 What then shall “we” be, when we shall see this? What is promised to us? “We shall be like Him, for we shall see Him as He is.” The tongue has done what it could, has sounded the words: let the rest be thought by the heart. For what has even John himself said in comparison of That which Is, or what can be said by us men, who are so far from being equal to his merits? Return we therefore to that unction of Him, return we to that unction which inwardly teaches that which we cannot speak: and because ye cannot at present see, let your part and duty be in desire.
The whole life of a good Christian is an holy desire. Now what you long for, you do not yet see: howbeit by longing, you are made capable, so that when that has come which you may see, you shall be filled. For just as, if you would fill a bag, and know how great the thing is that shall be given, you stretch the opening of the sack or the skin, or whatever else it be; you know how much you would put in, and see that the bag is narrow; by stretching you make it capable of holding more: so God, by deferring our hope, stretches our desire; by the desiring, stretches the mind; by stretching, makes it more capacious.
Let us desire therefore, my brethren, for we shall be filled. See Paul widening, as it were, his bosom, that it may be able to receive that which is to come. He says, namely, “Not that I have already received, or am already perfect: brethren, I deem not myself to have apprehended.” Then what are you doing in this life, if you have not yet apprehended? “But this one thing [I do]; forgetting the things that are behind, reaching forth to the things that are before, upon the strain I follow on unto the prize of the high calling.”
He says he reaches forth, or stretches himself, and says that he follows “upon the strain.” He felt himself too little to take in that “which eye has not seen, nor ear heard, neither has entered into the heart of man.” This is our life, that by longing we should be exercised. But holy longing exercises us just so much as we prune off our longings from the love of the world. We have already said, “Empty out that which is to be filled.” With good you are to be filled: pour out the bad.
Suppose that God would fill you with honey: if you are full of vinegar, where will you put the honey? That which the vessel bore in it must be poured out: the vessel itself must be cleansed; must be cleansed, albeit with labor, albeit with hard rubbing, that it may become fit for that thing, whatever it be. Let us say honey, say gold, say wine; whatever we say it is, being that which cannot be said, whatever we would fain say, It is called— God. And when we say “God,” what have we said? Is that one syllable the whole of that we look for? So then, whatever we have had power to say is beneath Him: let us stretch ourselves unto Him, that when He shall come, He may fill us. For “we shall be like Him; because we shall see Him as He is.”
Source: Homilies on the First Epistle of John (New Advent)