38 And in that the Church is thus described in the Song of Songs, "A garden enclosed is my sister, my spouse; a spring shut up, a fountain sealed, a well of living water; your plants are an orchard of pomegranates, with pleasant fruits;" I dare not understand this save of the holy and just,— not of the covetous, and defrauders, and robbers, and usurers, and drunkards, and the envious, of whom we yet both learn most fully from Cyprian's letters, as I have often shown, and teach ourselves, that they had baptism in common with the just, in common with whom they certainly had not Christian charity.
For I would that some one would tell me how they "crept into the garden enclosed and the fountain sealed," of whom Cyprian bears witness that they renounced the world in word and not in deed, and that yet they were within the Church. For if they both are themselves there, and are themselves the bride of Christ, can she then be as she is described "without spot or wrinkle," and is the fair dove defiled with such a portion of her members? Are these the thorns among which she is a lily, as it is said in the same Song? So far therefore, as the lily extends, so far does "the garden enclosed and the fountain sealed," namely, through all those just persons who are Jews inwardly in the circumcision of the heart (for "the king's daughter is all glorious within"), in whom is the fixed number of the saints predestined before the foundation of the world.
But that multitude of thorns, whether in secret or in open separation, is pressing on it from without, above number. "If I would declare them," it is said, "and speak of them, they are more than can be numbered." The number, therefore, of the just persons, "who are the called according to His purpose," of whom it is said, "The Lord knows them that are His," is itself "the garden enclosed, the fountain sealed, a well of living water, the orchard of pomegranates with pleasant fruits."
Of this number some live according to the Spirit, and enter on the excellent way of charity; and when they "restore a man that is overtaken in a fault in the spirit of meekness, they consider themselves, lest they also be tempted." And when it happens that they also are themselves overtaken, the affection of charity is but a little checked, and not extinguished; and again rising up and being kindled afresh, it is restored to its former course. For they know how to say, "My soul melts for heaviness: strengthen me according unto Your word." But when "in anything they be otherwise minded, God shall reveal even this unto them," if they abide in the burning flame of charity, and do not break the bond of peace.
But some who are yet carnal, and full of fleshly appetites, are instant in working out their progress; and that they may become fit for heavenly food, they are nourished with the milk of the holy mysteries, they avoid in the fear of God whatever is manifestly corrupt even in the opinion of the world, and they strive most watchfully that they may be less and less delighted with worldly and temporal matters. They observe most constantly the rule of faith which has been sought out with diligence; and if in anything they stray from it, they submit to speedy correction under Catholic authority, although, in Cyprian's words, they be tossed about, by reason of their fleshly appetite, with the various conflicts of phantasies.
There are some also who as yet live wickedly, or even lie in heresies or the superstitions of the Gentiles, and yet even then "the Lord knows them that are His." For, in that unspeakable foreknowledge of God, many who seem to be without are in reality within, and many who seem to be within yet really are without. Of all those, therefore, who, if I may so say, are inwardly and secretly within, is that "enclosed garden" composed, "the fountain sealed, a well of living water, the orchard of pomegranates, with pleasant fruits." The divinely imparted gifts of these are partly peculiar to themselves, as in this world the charity that never fails, and in the world to come eternal life; partly they are common with evil and perverse men, as all the other things in which consist the holy mysteries.
Source: On Baptism, Against the Donatists (New Advent)