32 In fact, St. Francis preached so well that his sermons were but "an exposition of the grace and power which dwelt within his own soul." His sermons, since they were largely made up of the teachings of the Bible and of the Fathers, became not only a source of sound doctrine but were agreeable and persuasive to his hearers as well by reason of the sweetness of the love which filled his heart. It is not surprising then that such a great number of heretics returned to the Church because of his work and that, following the guidance of such a teacher, so many of the faithful have, during the last three hundred years, attained a truly high degree of perfection.
Source: Rerum Omnium Perturbationem (Vatican.va)