6 And on this account, that one female, not only in the Spirit, but also in the flesh, is both a mother and a virgin. And a mother indeed in the Spirit, not of our Head, Which is the Saviour Himself, of Whom rather she was born after the Spirit: forasmuch as all, who have believed in Him, among whom is herself also, are rightly called “children of the Bridegroom:” but clearly the mother of His members, which are we: in that she wrought together by charity, that faithful ones should be born in the Church, who are members of That Head: but in the flesh, the mother of the Head Himself.
For it behooved that our Head, on account of a notable miracle, should be born after the flesh of a virgin, that He might thereby signify that His members would be born after the Spirit, of the Church a virgin: therefore Mary alone both in Spirit and in flesh is a mother and a virgin: both the mother of Christ, and a virgin of Christ; but the Church, in the Saints who shall possess the kingdom of God, in the Spirit indeed is altogether the mother of Christ, altogether a virgin of Christ: but in the flesh not altogether, but in certain a virgin of Christ, in certain a mother, but not of Christ.
Forsooth both faithful women who are married, and virgins dedicated to God, by holy manners, and charity out of a pure heart, and good conscience, and faith unfeigned, because they do the will of the Father, are after a spiritual sense mothers of Christ. But they who in married life give birth to (children) after the flesh, give birth not to Christ, but to Adam, and therefore run, that their offspring having been dyed in His Sacraments, may become members of Christ, forasmuch as they know what they have given birth to.
Source: On Holy Virginity (New Advent)