"So, brethren, let us long, because we are to be filled."

Ary Scheffer,
St. Augustine and his mother
St. Monica (1855),
The Louvre, Paris.

"The whole life of the good Christian is a holy longing [(sanctum desiderium)].  What you long for [(desideras)], as yet you do not see; but longing [(desiderando)] makes in you the room that shall be filled, when that which you are to see shall come.  When you would fill a purse, knowing how large a present it is to hold, you stretch wide [(extendis)] its cloth or leather:  knowing how much you are to put in it, and seeing that the purse is small, you extend [(extendit)] it to make more room.  So by withholding the vision God extends [(extendit)] the longing, through longing he makes the soul extend [(extendit)], by extending [(extendendo)] it he makes room in it.  So, brethren, let us long, because we are to be filled. . . . Let us stretch ourselves out [(extendamus)] towards him, that when he comes he may fill us full."

     St. Augustine, In epistulam Johannis ad Parthos tractatus decem 4.6 (407), trans. Burnaby (LCC 8, 290), underscoring mine.  Also WSA III.13; FC 92; NPNF 7.  Latin:  SC 75, 230, 232.  Also NBA 24/2; ed. Reale (1994); PL 35, cols. 2008-2009.
     This comes out in the Liturgy of the hours (Office of Readings for the Sixth Week in Ordinary Time?) as

The entire life of a good Christian is in fact an exercise of holy desire. You do not yet see what you long for, but the very act of desiring prepares you, so that when he comes you may see and be utterly satisfied. 

Suppose you are going to fill some holder or container, and you know you will be given a large amount. Then you set about stretching your sack or wineskin or whatever it is. Why? Because you know the quantity you will have to put in it and your eyes tell you there is not enough room. By stretching it, therefore, you increase the capacity of the sack, and this is how God deals with us. Simply by making us wait he increases our desire, which in turn enlarges the capacity of our soul, making it able to receive what is to be given to us. 

So, my brethren, let us continue to desire, for we shall be filled.

     Cf. letter 160 to Proba (c. 412), at FC 18, pp. 389-391:

our Lord and God does not need to have our will made known to him—He cannot but know it—but He wishes our desire to be exercised in prayer that we may be able to receive what He is preparing to give.  That is something very great, but we are too small and straitened to contain it.  Therefore it is said to us:  'Be enlarged, bear not the yoke with unbelievers.'  Thus we shall receive that which is so great, which eye hath not seen because it is not color, nor ear heard because it is not sound, nor hath it entered into the heart of man, because the heart of man has to enter into it; and we shall receive it in fuller measure in proportion as our hope is more strongly founded and our charity more ardent.
     . . . The more fervent the desire, the more worthy the effect which ensues.  And that is why the Apostle says:  'Desire without ceasing' [(Cf. 1 Thess 5:17)].  Let us, then, always desire this of the Lord God and always pray for it. . . .
     . . . Prayer is to be free of much speaking, but not of much entreaty, if the fervor and attention persist. . . .  to entreat much of Him whom we entreat is to knock by a long-continued and devout uplifting of the heart.