20 Whoever moreover wishes, and no one ought not so to wish, that his devotion should be worthy of her and perfect, should go further and strive might and main to imitate her example. It is a divine law that those only attain everlasting happiness who have by such faithful following reproduced in themselves the form of the patience and sanctity of Jesus Christ: "for whom He foreknew, He also predestined to be made conformable to the image of His Son; that He might be the first-born amongst many brethren" ( Romans viii., 29). But such generally is our infirmity that we are easily discouraged by the greatness of such an example: by the providence of God, however, another example is proposed to us, which is both as near to Christ as human nature allows, and more nearly accords with the weakness of our nature. And this is no other than the Mother of God. "Such was Mary," very pertinently points out St. Ambrose, "that her life is an example for all." And, therefore, he rightly concludes: "Have then before your eyes, as an image, the virginity and life of Mary from whom as from a mirror shines forth the brightness of chastity and the form of virtue" ( De Virginib . L. ii., c. ii.)
Source: Ad Diem Illum Laetissimum (Vatican.va)