The mystery of the Church, upon which the Second Vatican Council shed fresh light, has been repeatedly dealt with in numerous writings of theologians. While not a few of these studies have served to make this mystery more understandable, others, through the use of ambiguous or even erroneous language, have obscured Catholic doctrine, and at times have gone so far as to be opposed to Catholic faith even in fundamental matters.
To meet this situation, the bishops of several nations, conscious both of their duty of "keeping pure and intact the deposit of faith" and of their task of "proclaiming the Gospel unceasingly'", have, through concurring declarations, sought to protect the faithful entrusted to their care from the danger of error. In addition, the second General Assembly of the Synod of Bishops, in dealing with the ministerial priesthood, expounded a number of important points of doctrine regarding the consttution of the Church.
Likewise, the Sacred Congregation for the Doctrine of the Faith, whose task it is to "preserve the doctrine of faith and morals in the whole Catholic world"2, intends to gather together and explain a number of truths concerning the mystery of the Church which at the present time are being either denied or endangered. In this it will follow above all the lines laid down by the two Vatican Councils.