2 Knowing therefore these things, let us neither pride ourselves on children that are of good report, unless we have their virtue; nor upon noble fathers, unless we be like them in disposition. For it is possible, both that he who begot a man should not be his father, and that he who did not beget him should be. Therefore in another place also, when some woman had said, “Blessed is the womb that bare You, and the paps which You have sucked;” He said not, “The womb bare me not, neither did I suck the paps,” but this, “Yea rather, blessed are they that do the will of my Father.” Do you see how on every occasion He denies not the affinity by nature, but adds that by virtue? And His forerunner too, in saying, “O generation of vipers, think not to say, We have Abraham to our father,” means not this, that they were not naturally of Abraham, but that it profits them nothing to be of Abraham, unless they had the affinity by character; which Christ also declared, when He said, “If you were Abraham's children, you would do the works of Abraham;” not depriving them of their kindred according to the flesh, but teaching them to seek after that affinity which is greater than it, and more real.
This then He establishes here also, but in a manner less invidious, and more measured, as became Him speaking to His mother. For He said not at all, “She is not my mother, nor are those my brethren, because they do not my will;” neither did He declare and pronounce judgment against them; but He yet left in it their own power to choose, speaking with the gentleness that becomes Him.
“For he that does,” says He, “the will of my Father, this is my brother, and sister, and mother.”
Wherefore if they desire to be such, let them come this way. And when the woman again cried out, saying, “Blessed is the womb that bare You,” He said not, “She is not my mother,” but, “If she wishes to be blessed, let her do the will of my Father. For such a one is both brother, and sister, and mother.”
Oh honor! Oh virtue! Unto what a height does she lead up him that follows after her! How many women have blessed that holy Virgin, and her womb, and prayed that they might become such mothers, and give up all! What then is there to hinder? For behold, He has marked out a spacious road for us; and it is granted not to women only, but to men also, to be of this rank, or rather of one yet far higher. For this makes one His mother much more, than those pangs did. So that if that were a subject for blessing, much more this, inasmuch as it is also more real. Do not therefore merely desire, but also in the way that leads you to your desire walk thou with much diligence.
Source: Homilies on the Gospel of St. Matthew (New Advent)