- from CatholicCulture.org
Doctrines are what we, as Catholics, believe. They are truths which the Church tells us are things we must believe in. The presence of doctrines are one of the things which separate Catholics from most other denominations of Christianity. We have a clear set of beliefs which are held by all people who call themselves Catholics. These doctrines are defined for us by the Church through Sacred Scripture, Sacred Tradition and the teachings of the Magisterium. Because of this, doctrines do not change in the sense that we no longer believe in it. Instead, as our understanding grows, a doctrine may be added on to or our understanding of it may change in a way that allows for more insight. This differs from disciplines, which are practices of the Church. Examples of disciplines would be rules regarding fasting, Holy Days of Obligation and priestly celibacy. While these are things which must be followed by the faithful, they can also be changed by the Magisterium at any point in the future.