138 Therefore, out of this new diffusion throughout the world of the spirit of the Gospel, which is the spirit of Christian moderation and universal charity, We are confident there will come that longed-for and full restoration of human society in Christ, and that "Peace of Christ in the Kingdom of Christ," to accomplish which, from the very beginning of Our Pontificate, We firmly determined and resolved within Our heart to devote all Our care and all Our pastoral solicitude, and toward this same highly important and most necessary end now, you also, Venerable Brethren, who with Vs rule the Church of God under the mandate of the Holy Ghost, are earnestly toiling with wholly praiseworthy zeal in all parts of the world, even in the regions of the holy missions to the infidels. Let well-merited acclamations of praise be bestowed upon you and at the same time upon all those, both clergy and laity, who We rejoice to see, are daily participating and valiantly helping in this same great work, Our beloved sons engaged in Catholic Action, who with a singular zeal are undertaking with Us the solution of the social problems in so far as by virtue of her divine institution this is proper to and devolves upon the Church. All these We urge in the Lord, again and again, to spare no labors and let no difficulties conquer them, but rather to become day by day more courageous and more valiant. Arduous indeed is the task which We propose to them, for We know well that on both sides, both among the upper and the lower classes of society, there are many obstacles and barriers to be overcome. Let them not, however, lose heart; to face bitter combats is a mark of Christians, and to endure grave labors to the end is a mark of them who, as good soldiers of Christ, follow Him closely.
Source: Quadragesimo Anno (Vatican.va)