26 Those who are sent to different nations in order to be good ministers of Christ, should he nourished with the "words of faith and with good doctrine" (1 Tim. 4:6), which they should draw principally from the Sacred Scriptures, studying the mystery of Christ, whose heralds and witnesses they will be. Therefore, all missionaries - priests, Brothers, Sisters, and lay folk - each according to their own state, should be prepared and trained, lest they be found unequal to the demands of their future work. From the very beginning, their doctrinal training should be so planned that it takes in both the universality of the Church and the diversity of the world's nations. This holds for all of their studies by which they are prepared for the exercise of the ministry, as also for the other studies which it would be useful for them to learn, that they may have a general knowledge of the peoples, cultures, and religions; not only a knowledge that looks to the past, but one that considers the present time. For anyone who is going to encounter another people should have a great esteem for their patrimony and their language and their customs. It is very necessary for the future missionary to devote himself to missiological studies: that is, to know the teachings and norms of the Church concerning missionary activity, to know along what roads the heralds of the Gospel have run in the course of the centuries, and also what is the present condition of the missions, and what methods are considered more effective at the present time. But even though this entire training program is imbued with pastoral solicitude, a special and organized apostolic training ought to be given, by means of both teaching and practical exercises. Brothers and Sisters, in great numbers, should be well instructed and prepared in the catechetical art, that they may collaborate still better in the apostolate. Even those who take part in missionary activity only for a time have to be given a training which is suited to their condition. All these different kinds of formation should be completed in the lands to which they are sent, so that the missionaries may have a more thorough knowledge of the history, social structures, and customs of the people; that they may have an insight into their moral order and their religious precepts, and into the secret notions which, according to their sacred tradition, they have formed concerning God, the world and man. Let the missionaries learn the languages to such a degree that they can use them in a fluent and polished manner, and so find more easy access to the minds and the hearts of men. Furthermore, they should be properly introduced into special pastoral problems. Some should be more thoroughly prepared in missiological institutes or in other faculties or universities, so that they may be able to discharge special duties more effectively and be a help, by their learning, to other missionaries in carrying on the mission work, which especially in our time presents so many difficulties and opportunities. It is moreover highly desirable that the regional episcopal conferences should have available an abundance of such experts, and that they should make fruitful use of their knowledge and experience in the necessities of their office. Nor should there be wanting some who are perfectly skilled in the use of practical instruments and the means of social communication, the importance of which should be highly appreciated by all.
Source: Ad Gentes (Vatican.va)