8 It is true that not all understand fully the necessity of this holy apostolate of the laity, although from Our first Encyclical, Ubi Arcano Dei , We declared that this appertains undeniably to the pastoral ministry and to Christian life. But since, as We have already indicated, We are addressing Ourselves to pastors who must regain a sorely tried and to a certain extent dispersed flock, today more than ever before We recommend that you make use of those secular people to whom, as living stones of the Holy House of God, St. Peter attributes a profound dignity which makes them in a certain manner participants in a holy and regal priesthood (1 Peter ii.9). In fact, every Christian conscious of his dignity and his responsibility as a son of the Church and a member of the Mystical Body of Christ - Multi Unum Corpus Sumus in Christo Singuli Autem Alter Alterius Membra (So we being many, are one body in Christ, and every one members one of another) ( Romans xii. 5) - cannot do less than recognize that between the members of this body there must exist a reciprocal communication of life and solidarity of interests. Hence the duty of each in the order of life and the increase of the whole organism in aedificationem Corporis Chrisn : hence the efficacious contribution of each member toward the glorification of the Head and of His Mystical Body ( Ephesians iv. 12-16). From these clear and simple principles, what consoling deductions, what luminous directives arise for many souls still uncertain and diffident, but desirous of orientating their ardor! What incitements to contribute to the spread of the Kingdom of Christ and to the salvation of souls!
Source: Firmissimam Constantiam (Vatican.va)