34 Anyone who deems himself a Christian must know that he is bound by his conscience to the basic, imperative duty of bearing witness to the truth in which he believes and to the grace which has transformed his soul. A great Father of the Church has said: "He (Christ) left us on earth in order that we should become like beacons of light and teachers unto others; that we might act like leaven, move among men like angels, be like men unto children, and like spiritual men unto animal men, in order to win them over, and that we may be like seed, and bear abundant fruits. There would be no need for sermons, if our lives were shining; there would be no need for words, if we bore witness with our deeds. There would be no more pagans, if we were true Christians." (60)
Source: Princeps Pastorum (Vatican.va)