35 Indeed the herald of evangelical truth must of necessity be a 12man of God not only because he has been divinely called, but also because he has vowed himself to Him fully and perpetually. "Truly" - as our predecessor Benedict XV teaches in the admirable apostolic letter Maximum illud - "he must be a man of God, who preaches God; he must hate sin, who commands to hate sin. More progress will be made in preaching the faith by example than by words, especially among infidels who are led by the senses rather than by reason."
Source: Saeculo Exeunte Octavo (Vatican.va)