"The thought of the Church is essentially a traditional thought, and the
living magisterium…prepares to give to immutable truth a
new expression which shall be in harmony with
the circumstances of the day and within reach of contemporary minds. Revealed truth has sometimes found definitive formulas from the earliest times; then the living magisterium has only had to preserve and explain them and put them in circulation. Sometimes attempts have been made to express this truth, without success…it has [sometimes] been distorted so as to be scarcely recognizable, so closely mingled with error that it becomes difficult to separate them. When the Church studies the ancient monuments of her faith she casts over the past the reflection of
her living and present thought and by some sympathy of the truth of today with that of yesterday she succeeds in recognizing through the obscurities and inaccuracies of ancient formulas the portions of traditional truth, even when they are mixed with error. The Church is also (as regards religious and moral doctrines) the best interpreter of truly traditional documents; she recognizes as by instinct what ***belongs to the current of her living thought and distinguishes it from the foreign elements which may have become mixed with it in the course of centuries. ***
The living magisterium, therefore, makes extensive use of documents of the past, but it does so while judging and interpreting, gladly finding in them its present thought, but likewise, when needful, distinguishing its present thought from what is
traditional only in appearance.
CATHOLIC ENCYCLOPEDIA: Tradition and Living Magisterium