3 Indeed, as all know, for the spread of God's kingdom Jesus Christ used no other weapon than the preaching of the Gospel, that is, the living voice of His heralds, who diffused everywhere the celestial doctrine. "Teach," he said, "all nations." "Preach the Gospel to every creature." Accordingly, from the preaching of the Apostles, and especially of St. Paul, it came to pass, that preaching being followed up with the doctrine and discipline of the Fathers and afterwards of the Doctors, men's minds were enlightened with the light of truth and conceived a love for all the virtues. Following the same lines in his work for the salvation of souls. Dominic proposed to himself and to all his followers "to hand to others what they had contemplated." For this reason, in addition to the duty of cultivating poverty, innocence of life, and religious discipline, he commanded his Order in a strict and solemn manner to be zealous in the study of Christian doctrine and the preaching of the truth.
Source: Fausto Appetente Die (Vatican.va)