The faithful as a whole cannot err in faith, because Jesus promised his disciples that he would send them the Spirit of truth and keep them in the truth (Jn 14:17).
Just as the disciples believed Jesus with their whole heart, a Christian can rely completely on the Church when he asks about the way to life. Since Jesus Christ himself gave his apostles the commission to teach, the Church has a teaching authority (The Magisterium) and must not remain silent. Although individual members of the Church can err and even make serious mistakes, the Church as a whole can never fall away from God’s truth. The Church carries through the ages a living truth that is greater than herself. We speak about a depositum fidei, a deposit of faith that is to be preserved. If such a truth is publicly disputed or distorted, the Church is called upon to clarify again “what has always and everywhere been believed by all” (St. Vincent of Lerins, d. 450).