143 These Our Beloved Sons who are chosen for so great a work, We earnestly exhort in the Lord to give themselves wholly to the training of the men committed to their care, and in the discharge of this eminently priestly and apostolic duty to make proper use of the resources of Christian education by teaching youth, forming Christian organizations, and founding study groups guided by principles in harmony with the Faith. But above all, let them hold in high esteem and assiduously employ for the good of their disciples that most valuable means of both personal and social restoration which, as We taught in Our Encyclical, Mens Nostra , is to be found in the Spiritual Exercises. In that Letter We expressly mentioned and warmly recommended not only the Spiritual Exercises for all the laity, but also the highly beneficial Workers' Retreats. For in that school of the spirit, not only are the best of Christians developed but true apostles also are trained for every condition of life and are enkindled with the fire of the heart of Christ. From this school they will go forth as did the Apostles from the Upper Room of Jerusalem, strong in faith, endowed with an invincible steadfastness in persecution, burning with zeal, interested solely in spreading everywhere the Kingdom of Christ.
Source: Quadragesimo Anno (Vatican.va)