But look here, my brothers and sisters, concentrate more, I beg you, on what follows, concentrate more on what Christ the Lord said as he stretched out his hand over his disciples: This is my mother and these are my brothers; and whoever does the will of my Father who sent me, that person is a brother to me and a sister and a mother (Mt 12:49-50). Didn’t the Virgin Mary do the will of the Father? I mean, she believed by faith, she conceived by faith, she was chosen to be the one from whom salvation in the very midst of the human race would be born for us, she was created by Christ before Christ was created in her. Yes, of course, holy Mary did the will of the Father. And therefore it means more for Mary to have been a disciple of Christ than to have been the mother of Christ. It means more for her, an altogether greater blessing, to have been Christ’s disciple than to have been Christ’s mother. That is why Mary was blessed, because even before she gave him birth, she bore her teacher in her womb.
Just see if it isn’t as I say. While the Lord was passing by, performing divine miracles, with the crowds following him, a woman said: Fortunate is the womb that bore you. And how did the Lord answer, to show that good fortune is not really to be sought in mere family ties? Rather blessed are those who hear the word of God and keep it (Lk 11:27-28). So that is why Mary, too, is blessed, because she heard the word of God and kept it. She kept truth safe in her mind even better than she kept flesh safe in her womb. Christ is truth, Christ is flesh; Christ as truth was in Mary’s mind, Christ as flesh in Mary’s womb; that which is in the mind is greater than what is carried in the womb.
Mary is holy, Mary is blessed, but the Church is something better than the Virgin Mary. Why? Because Mary is part of the Church, a holy member, a quite exceptional member, the supremely wonderful member, but still a member of the whole body. That being so, it follows that the body is something greater than the member. The Lord is the head, and the whole Christ is head and body. How shall I put it? We have a divine head, we have God as our head.
Source: St. Augustine, Sermon 72/A, 7