57 To sum up, then, We may lay it down as a general and lasting law that working men's associations should be so organized and governed as to furnish the best and most suitable means for attaining what is aimed at, that is to say, for helping each individual member to better his condition to the utmost in body, soul, and property. It is clear that they must pay special and chief attention to the duties of religion and morality, and that social betterment should have this chiefly in view; otherwise they would lose wholly their special character, and end by becoming little better than those societies which take no account whatever of religion. What advantage can it be to a working man to obtain by means of a society material well-being, if he endangers his soul for lack of spiritual food? "What doth it profit a man, if he gain the whole world and suffer the loss of his soul?"(39)This, as our Lord teaches, is the mark or character that distinguishes the Christian from the heathen. "After all these things do the heathen seek . . . Seek ye first the Kingdom of God and His justice: and all these things shall be added unto you."(40)Let our associations, then, look first and before all things to God; let religious instruction have therein the foremost place, each one being carefully taught what is his duty to God, what he has to believe, what to hope for, and how he is to work out his salvation; and let all be warned and strengthened with special care against wrong principles and false teaching. Let the working man be urged and led to the worship of God, to the earnest practice of religion, and, among other things, to the keeping holy of Sundays and holy days. Let him learn to reverence and love holy Church, the common Mother of us all; and hence to obey the precepts of the Church, and to frequent the sacraments, since they are the means ordained by God for obtaining forgiveness of sin and for leading a holy life.
Source: Rerum Novarum (Vatican.va)