11 But you, beloved, who possess this faith, or who have begun now newly to have it, let it be nourished and increase in you. For as things temporal have come, so long before foretold, so will things eternal also come, which are promised. Nor let them deceive you, either the vain heathen, or the false Jews, or the deceitful heretics, or also within the Catholic (Church) itself evil Christians, enemies by so much the more hurtful, as they are the more within us. For, lest on this subject also the weak should be troubled, divine prophecy has not been silent, where in the Song of Songs the Bridegroom speaking unto the Bride, that is, Christ the Lord unto the Church, says, “As a lily in the midst of thorns, so is my best Beloved in the midst of the daughters.” He said not, in the midst of them that are without; but, “in the midst of daughters.
Whoso has ears to hear, let him hear:” and while the net which is cast into the sea, and gathers together all kinds of fishes, as says the holy Gospel, is being drawn unto the shore, that is, unto the end of the world, let him separate himself from the evil fishes, in heart, not in body; by changing evil habits, not by breaking sacred nets; lest they who now seem being approved to be mingled with the reprobate, find, not life, but punishment everlasting, when they shall begin on the shore to be separated.
Source: Concerning Faith of Things Not Seen (New Advent)