22 Moreover, the foreign missionary, because of his imperfect knowledge of the language often finds himself embarrassed when he attempts to express his thoughts with the result that the force and efficacy of his preaching are thereby greatly weakened. In addition to the aforementioned difficulties there are others which must always be taken into account, notwithstanding the fact that these difficulties are of rare occurrence and can oftentimes be overcome easily. Let us suppose, for example, that either because of the fortunes of war, or because of certain political happenings in a mission field, the ruling government is changed in that territory and that the new government decrees or requests that the missionaries of a certain nationality be expelled; or let us suppose - something which rarely, if ever, occurs - that the inhabitants of a particular territory, having reached a fairly high degree of civilization and at the same time a corresponding development in civic and social life, and desiring to become free and independent, should drive away from their country the governor, the soldiers, the missionaries of the foreign nation to whose rule they are subject. All this, of course, cannot be done without violence. Everyone can see what great harm would accrue to the Church in that land in the circumstances, unless a native clergy had been spread beforehand throughout the country like a network and were, by consequence, in a position to provide adequately for the population which had been converted to Christ.
Source: Rerum Ecclesiae (Vatican.va)