The faithful, clerics or lay, are witness to the truth after the example of Jesus, who claimed before Pilate that
he came to bear witness to the truth (Jn 18,37)
after the example of all martyrs, lay people are Collaborateors with the truth
(Cf. 3 Jn 8: Apostolicam Actuositatem,
6)
"The will of the father is that we recognize in every human being Christ our brother, and that we love each one for his
good, in word and deed, thus bearing witness to the truth."
(Gaudium et Ses, 93)
"All the laity must cooperate in the spreading and growth of the Kingdom of Christ in the world ... Every lay person
ought to be, before the world, a witness of the resurrection and of the life of the Lord Jesus and a sign of the Living God."
(Lumen Gentium, 35 and 38)
