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.
Development means that each thing expands to be itself, while alteration means that a thing is changed from one thing into another.
St. Vincent explains that development is always good, but alteration is not.
The religion of souls should follow the law of development of bodies. Though bodies develop and unfold their component parts with the passing of the years, they always remain what they were. There is a great difference between the flower of childhood and the maturity of age, but those who become old are the very same people who were once young. Though the condition and appearance of one and the same individual may change, it is one and the same nature, one and the same person.
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.
If, however, the human form were to turn into some shape that did not belong to its own nature, or even if something were added to the sum of its members or subtracted from it, the whole body would necessarily perish or become grotesque or at least be enfeebled. In the same way, the doctrine of the Christian religion should properly follow these laws of development, that is, by becoming firmer over the years, more ample in the course of time, more exalted as it advances in age.
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.