17 In the third place the "spirit of prayer," as it is called, is necessary, the Apostle tells us, for the preacher. No sooner was he himself called to the apostolate than he began his supplications to God. "For behold he prayeth." [ Acts ix:11] For it is not by pouring forth a copious stream of words, not by using subtle arguments, not by delivering violent harangues, that the salvation of souls is effected. The preacher who is content with those means is nothing but "sounding brass or a tinkling cymbal." [I Cor . xiii:1] What gives a man's words life and vigor and makes them promote wonderfully the salvation of souls is Divine grace: "God gave the increase." [I Cor . iii:6] But the grace of God is not gained by study and practice: it is won by prayer. Therefore he who is little given to prayer or neglects it altogether, vainly spends his time and labor in preaching, for in God's sight his sermons profit neither himself nor those who hear him.
Source: Humani Generis Redemptionem (Vatican.va)