5 Furthermore, our Lord often made use of this method in forming the preachers of the Gospel. For the Divine Master Himself, not content with having spent long years in the domestic retreat of Nazareth, before he shone forth in full light before the nations, and taught them heavenly things by his word, chose to spend full forty days in desert wilderness. Nay more, in the midst of his evangelical labours, he was wont to invite his Apostles to the friendly silence of retreat: "Come apart into a desert place, and rest a little," and when he left this earth of sorrows to go to heaven, he willed that these same Apostles and his disciples should be polished and perfected in the upper chamber at Jerusalem, where for the space of ten days "persevering with one mind in prayer" they were made worthy to receive the Holy Spirit: surely a memorable retreat, which first foreshadowed the Spiritual Exercises; from which the church came forth endowed with virtue and perpetual strength; and in which, in the presence of the Virgin Mary Mother of God, and aided by her patronage, those also were instituted whom we may rightly call precursors of the Catholic Action.
Source: Mens Nostra (Vatican.va)