7 In the first place, both in sermons and by your writings, strive to have introduced and gradually to extend the pious custom of praying "the Lord of the harvest, that he send forth laborers into his harvest" ( Matt . ix, 38) and of asking for the heathen the light of the Holy Spirit and the grace of God. We say that these prayers should become habitual for it is evident that prayers said in this manner cannot but have more efficacy before the seat of Divine Mercy than prayers said but once or only occasionally. Even though the missionaries labor most zealously, though they work and toil and go so far as to lay down their very lives in order to bring to the pagans a knowledge of the Catholic religion, though they employ every means known to human ingenuity and spare themselves in nothing, all this will avail them nothing, all their efforts will go for naught, if God by His grace does not touch the hearts of the heathen in order to soften and attract them to Himself.
Source: Rerum Ecclesiae (Vatican.va)