Lumen Gentium rejected a notion of a clergy-first institution. One of the consequences of starting with the People of God is that naturally the duties and responsibilities of that people are also emphasized. Every believer is called to witness to the Word:
6. The renewal of the Church is also achieved through the witness offered by the lives of believers: by their very existence in the world, Christians are called to radiate the word of truth that the Lord Jesus has left us. The Council itself, in the Dogmatic Constitution Lumen Gentium, said this: While “Christ, ‘holy, innocent and undefiled’ (Heb 7:26) knew nothing of sin (cf. 2 Cor 5:21), but came only to expiate the sins of the people (cf. Heb 2:17)… the Church … clasping sinners to its bosom, at once holy and always in need of purification, follows constantly the path of penance and renewal. The Church, ‘like a stranger in a foreign land, presses forward amid the persecutions of the world and the consolations of God’, announcing the cross and death of the Lord until he comes (cf. 1 Cor 11:26). But by the power of the risen Lord it is given strength to overcome, in patience and in love, its sorrow and its difficulties, both those that are from within and those that are from without, so that it may reveal in the world, faithfully, although with shadows, the mystery of its Lord until, in the end, it shall be manifested in full light.”(Lumen Gentium, 8.)
In a word, grace. God’s grace enables the Church and its believers to give that shadowed, imperfect witness.