28 Go on therefore in your course, and run with perseverance, in order that you may obtain; and by pattern of life, and discourse of exhortation, carry away with you into this same your course, whomsoever you shall have had power. Let there not bend you from this earnest purpose, whereby ye excite many to follow, the complaint of vain persons, who say, How shall the human race subsist, if all shall have been continent? As though it were for any other reason that this world is delayed, save that the predestined number of the Saints be fulfilled, and were this the sooner fulfilled, assuredly the end of the world would not be put off.
Nor let it stay you from your earnest purpose of persuading others to the same good you have, if it be said to you, Whereas marriage also is good, how shall there be all goods in the Body of Christ, both the greater, forsooth, and the lesser, if all through praise and love of continence imitate? In the first place, because with the endeavor that all be continent, there will still be but few, for “not all receive this word.” But forasmuch as it is written, “Whoso can receive, let him receive;” then do they receive who can, when silence is not kept even toward those who cannot.
Next, neither ought we to fear lest haply all receive it, and some one of lesser goods, that is, married life, be wanting in the body of Christ. For if all shall have heard, and all shall have received, we ought to understand that this very thing was predestinated, that married goods already suffice in the number of those members which so many have passed out of this life. For neither now, if all shall have been continent, will they give the honor of the continent to those who have already borne into the garners of the Lord the fruit thirty-fold, if that be understood of married good.
Therefore all these goods will have there their place, although from this time no woman wish to be married, no man wish to marry a wife. Therefore without anxiety urge on whom you can, to become what you are; and pray with watchfulness and fervor, that by the help of the Right Hand of the Most High, and by the abundance of the most merciful grace of the Lord, you may both persevere in that which you are, and may make advances unto that which you shall be.
Source: On the Good of Widowhood (New Advent)