84 The accomplishment of this task of regeneration, by adapting her means to the altered conditions of the times and to the new needs of the human race, is an essential and maternal office of the Church. Committed to her by her Divine Founder, the preaching of the Gospel, by which is inculcated to men truth, justice and charity and the endeavor to implant its precepts solidly in mind and conscience, is the most noble and most fruitable work for peace. That mission would seem as if it ought to discourage by its very grandeur the hearts of those who make up the Church Militant. But that cooperation in the spread of the Kingdom of God which in every century is effected in different ways, with varying instruments, with manifold hard struggles, is a command incumbent on everyone who has been snatched by Divine Grace from the slavery of Satan and called in Baptism to citizenship of the Kingdom of God.
Source: Summi Pontificatus (Vatican.va)