28 We, however, are certain that no one could ever have been warranted in separating himself from the communion of all nations, because every one of us looks for the marks of the Church not in his own righteousness, but in the Divine Scriptures, and beholds it actually in existence, according to the promises. For it is of the Church that it is said,“As the lily among thorns, so is my love among the daughters;” which could be called on the one hand “thorns” only by reason of the wickedness of their manners, and on the other hand “daughters” by reason of their participation in the same sacraments. Again, it is the Church which says, “From the end of the earth have I cried unto You when my heart was overwhelmed;” and in another Psalm, “Horror has kept me back from the wicked that forsake Your law;” and, “I beheld the transgressors, and was grieved.” It is the same which says to her Spouse: “Tell me where You feed, where You rest at noon: for why should I be as one veiled beside the flocks of Your companions?” This is the same as is said in another place: “Make known to me Your right hand, and those who are in heart taught in wisdom;” in whom, as they shine with light and glow with love, You rest as in noontide; lest perchance, like one veiled, that is, hidden and unknown, I should run, not to Your flock, but to the flocks of Your companions, i.e. of heretics, whom the bride here calls companions, just as He called the thorns “daughters,” because of common participation in the sacraments: of which persons it is elsewhere said: “You were a man, mine equal, my guide, my acquaintance, who took sweet food together with me; we walked unto the house of God in company. Let death seize upon them, and let them go down quick into hell,” like Dathan and Abiram, the authors of an impious schism.
29. It is to the Church also that the answer is given immediately after in the passage quoted above: “If you know not yourself, O you fairest among women, go your way forth by the footsteps of the flocks, and feed your kids beside the shepherds' tents.” Oh, matchless sweetness of the Bridegroom, who thus replied to her question: “If you know not yourself,” He says; as if He said, “Surely the city which is set upon a mountain cannot be hid; and therefore, 'You are not as one veiled, that you should run to the flocks of my companions.' For I am the mountain established upon the top of the mountains, unto which all nations shall come. 'If you know not yourself,' by the knowledge which you may gain, not in the words of false witnesses, but in the testimonies of My book; 'if you know not yourself,' from such testimony as this concerning you: 'Lengthen your cords, and strengthen your stakes: for you shall break forth on the right hand and on the left; and your seed shall inherit the Gentiles, and make the desolate cities to be inhabited. Fear not, for you shall not be ashamed; neither be confounded, for you shall not be put to shame: for you shall forget the shame of your youth, and shall not remember the reproach of your widowhood any more: for your Maker is your husband, the Lord of hosts is His name, and your Redeemer the Holy One of Israel; the God of the whole earth shall He be called.' 'If you know not yourself,' O you fairest among women, from this which has been said of you, 'The King has greatly desired your beauty,' and 'instead of your fathers shall be your children, whom you may make princes upon the earth:' if, therefore, 'you know not yourself,' go your way forth: I do not cast you forth, but 'go your way forth,' that of you it may be said, 'They went out from us, but they were not of us.' 'Go your way forth' by the footsteps of the flocks, not in My footsteps, but in the footsteps of the flocks; and not of the one flock, but of flocks divided and going astray. 'And feed your kids,' not as Peter, to whom it is said, 'Feed My sheep;' but, 'Feed your kids beside the shepherds' tents,' not beside the tent of the Shepherd, where there is 'one fold and one Shepherd.'” But the church knows herself, and thereby escapes from that lot which has befallen those who did not know themselves to be in her.
30. The same [Church] is spoken of, when, in regard to the fewness of her numbers as compared with the multitude of the wicked, it is said: “Strait is the gate and narrow is the way which leads unto life, and few there be that find it.” And again, it is of the same Church that it is said with respect to the multitude of her members: “I will multiply your seed as the stars of heaven, and as the sand which is upon the sea-shore.” For the same Church of holy and good believers is both small if compared with the number of the wicked, which is greater, and large if considered by itself; “for the desolate has more sons than she which has an husband,” and “many shall come from the east and from the west, and shall sit down with Abraham, and Isaac, and Jacob, in the kingdom of God.” God, moreover, presents unto Himself a “numerous people, zealous of good works.” And in the Apocalypse, many thousands “which no man can number,” from every tribe and tongue, are seen clothed in white robes, and with palms of victory. It is the same Church which is occasionally obscured, and, as it were, beclouded by the multitude of offenses, when sinners bend the bow that they may shoot under the darkened moon at the upright in heart. But even at such a time the Church shines in those who are most firm in their attachment to her. And if, in the Divine promise above quoted, any distinct application of its two clauses should be made, it is perhaps not without reason that the seed of Abraham was compared both to the “stars of heaven,” and to “the sand which is by the sea-shore:” that by “the stars” may be understood those who, in number fewer, are more fixed and more brilliant; and that by “the sand on the sea-shore” may be understood that great multitude of weak and carnal persons within the Church, who at one time are seen at rest and free while the weather is calm, but are at another time covered and troubled under the waves of tribulation and temptation.
Source: Letters (New Advent)