The body of the faithful is strictly speaking the Ecclesia docta (the Church taught), in contrast with the Ecclesia docens (the teaching Church), which consists of the pope and the bishops. When there is question, therefore, of the official teaching of religious doctrine, the laity is neither competent nor authorized to speak in the name of God and the Church (cap. xii et sq., lib. V, tit. vii, "de haereticis"). Consequently they are not allowed to preach in church, or to undertake to defend the Catholic doctrine in public discussions with heretics. But in their private capacity, they may most lawfully defend and teach their religion by word and writing, while submitting themselves to the control and guidance of ecclesiastical authority. Moreover, they may be appointed to give doctrinal instruction more or less officially, or may even become the defenders of Catholic truth. Thus they give excellent help to the clergy in teaching catechism, the lay masters in our schools give religious instruction, and some laymen have received a missio canonica, or due ecclesiastical authorization, to teach the religious sciences in universities and seminaries; the important point in this, as in other matters, is for them to be submissive to the legitimate teaching authority.
---Catholic Encylopedia, 1910.