10 Moreover, when we stop to consider the vast territory which remains as yet unopened to the blessings of Christian culture, the immense number of those who are still deprived of the fruits of the Redemption, or the obstacles and difficulties which beset and impede the best efforts of the missionary, it is absolutely necessary that the bishops and faithful work together in order that the number of the ambassadors of Christ be increased and multiplied. If there should be in your dioceses, any young men, seminarians, or priests who seem called by God to this sublime apostolate, far from putting obstacles in their way, you should encourage them both by your favor and authority in their leanings and desires. Though you are surely permitted in regard to these vocations "to try the spirits if they be of God" (I John iv, 1) still if you are convinced that their holy resolution springs from and is fostered by the Spirit of God, then neither scarcity of priests, nor any special need of your own diocese ought to discourage you or keep you from giving your consent to the vocation of anyone, since your own faithful have at hand, if We may use the phrase, the means to salvation, and are less further removed from them than the heathens, especially those who are still savages or are only semicivilized. If, therefore, the occasion should arise, suffer patiently for the love of Christ and of souls, the loss of one of your clergy, if indeed it can rightly be called as loss.
Source: Rerum Ecclesiae (Vatican.va)