From The Catechism of the Catholic Church:

899 Lay believers are in the front line of Church life; for them the Church is the animating principle of human society. Therefore, they in particular ought to have an ever-clearer consciousness not only of belonging to the Church, but of being the Church, that is to say, the community of the faithful on earth under the leadership of the Pope, the common Head, and of the bishops in communion with him. They are the Church.

900 Since, like all the faithful, lay Christians are entrusted by God with the apostolate by virtue of their Baptism and Confirmation, they have the right and duty, individually or grouped in associations, to work so that the divine message of salvation may be known and accepted by all men throughout the earth. This duty is the more pressing when it is only through them that men can hear the Gospel and know Christ. Their activity in ecclesial communities is so necessary that, for the most part, the apostolate of the pastors cannot be fully effective without it.