12 Let us return and see what this John says: “He that has the bride is the bridegroom;” she is not my bride. And do you not rejoice in the marriage? Yea, says he, I do rejoice: “But the friend of the bridegroom, who stands and hears him, rejoices greatly because of the voice of the bridegroom.” Not because of my own voice, says he, do I rejoice, but because of the Bridegroom's voice. I am in the place of hearer; He, of speaker: I am as one that must be enlightened, He is the light; I am as the ear, He is the word.
Therefore the friend of the Bridegroom stands and hears Him. Why stands? Because he falls not. How falls not? Because he is humble. See him standing on solid ground; “I am not worthy to loose the latchet of His shoe.” You do well to be humble; deservedly you do not fall; deservedly you stand, and hear Him, and rejoicest greatly for the Bridegroom's voice. So also the apostle is the Bridegroom's friend; he too is jealous, not for himself, but for the Bridegroom. Hear his voice when he is jealous: “I am jealous over you,” said he, “with the jealousy of God:” not with my own, nor for myself, but with the jealousy of God.
Why? How? Over whom are you jealous, and for whom? “For I have espoused you to one husband, to present a chaste virgin to Christ.” Why do you fear, then? Why are you jealous? “I fear,” says he, “lest, as the serpent beguiled Eve by his subtlety, so your minds should be corrupted from the chastity which is in Christ.” The whole Church is called a virgin. You see that the members of the Church are various, that they are endowed with and do rejoice in various gifts: some men wedded, some women wedded; some are widowers who seek no more to have wives, some are widows who seek no more to have husbands; some men preserve continence from their youth, some women have vowed their virginity to God: various are the gifts, but all these are one virgin.
Where is this virginity, for it is not in the body. It belongs to few women; and if virginity can be said of men, to few men in the Church belongs a holy integrity even of body; yet one such is a more honorable member. Other members, however, preserve virginity, not in body, but all in mind. What is the virginity of the mind? Entire faith, firm hope, sincere charity. This is the virginity which he, who, was jealous for the Bridegroom, feared might be corrupted by the serpent. For, just as the bodily member is marred in a certain part, so the seduction of the tongue defiles the virginity of the heart. Let her who does not desire without cause to keep virginity of body, see to it that she be not corrupted in mind.
Source: Tractates on the Gospel of John (New Advent)