Nicene and Post-Nicene Fathers, Vol. III:
Moral Treatises of St. Augustin: Section 5

5. There is, therefore, no reason why the virgins of God be sad, because themselves also cannot, keeping their virginity, be mothers of the flesh. For Him alone could virginity give birth to with fitting propriety, Who in His Birth could have no peer. However, That Birth of the Holy Virgin is the ornament of all holy virgins; and themselves together with Mary are mothers of Christ, if they do the will of His Father. For Mary also is on this account the Mother of Christ in a way more full of praise and blessing, according to His sentence mentioned above. “Whosoever doeth the will of my Father Who is in heaven, that one is to Me brother, and sister, and mother.” All these degrees of nearness of kin to Himself, He shows forth in a spiritual manner, in the People whom He hath redeemed: as brothers and sisters He hath holy men and holy women, forasmuch as they all are co-heirs in the heavenly inheritance. His mother is the whole Church, because she herself assuredly gives birth to His members, that is, His faithful ones. Also His mother is every pious soul, doing the will of His Father with most fruitful charity, in them of whom it travaileth, until Himself 2032 be formed in them. Mary, therefore, doing the will of God, after the flesh, is only the mother of Christ, but after the Spirit she is both His sister and mother.

Footnotes

Gal. 4.19Gal. iv. 19