At times it can be confusing to see the Catholic Church make new proclamations of what she believes.
The dogma of the Assumption of Mary is a prime example, as it was officially defined very recently: in 1950.
It may thus seem like a "new" doctrine, when in reality it is only the Church defining something she has believed for centuries.
A 4th-century monk, St. Vincent of Lérins, gives an easily understood example in a letter that is featured in the Church's Office of Readings.
St. Vincent explains that development is always good, but alteration is not.
The key is that the Church remains the same, though may look differently than it did when it was a "child."
Alteration, on the other hand, would disfigure the Church and change doctrine to be something completely different.
St. Vincent's letter offers the perfect criteria for whenever Church leaders assemble to discuss Church matters. Development is good and praiseworthy, but alteration, adding to or subtracting from beliefs, is always to be avoided.
