The Immaculata appears in this world, without the least stain of sin, the masterpiece of God’s hand, full of grace. God, the most Holy Trinity, beholds the lowliness (i.e. the humility, the root of all Her other virtues) of His handmaid, and ‘does great things’ for Her (cf. Lk 1:49). God the Father gives Her His own Son to be Her Son; God the Son descends into Her womb; and God the Holy Spirit forms the body of Christ in the womb of this most pure virgin. ‘And the Word was made flesh’ (Jn 1:14). The Immaculata becomes the Mother of God. The fruit of the love of God in His Trinitarian life and of Mary, the Immaculate, is Christ, the God-Man. Henceforth all the sons of God must be modelled after this first Son of God, the God-Man, the infinite One. They must reproduce His traits; by imitating Christ souls reach sanctity.
This is the union brought about by the spousal love of the soul for Christ, through its resemblance to Him, and by God’s action. But if anyone does not wish to have Mary Immaculate for his mother, he will not have Christ for his Brother.
Since the first born Son, the God-Man, was conceived only with specific consent of the most Blessed Virgin, the same must hold true for all men, who must be conformed to their first model in all things.
(From Sketches for a Book, 1940)