Placing God first inevitably means believing more than the fact that He exists. Hence, fulfillment of the First Commandment also means believing the truth of who He has revealed Himself to be and the implications of that revelation for our lives.

So what if someone believes that God exists but doesn’t believe something He’s revealed? It might mean committing the sin of heresy or even apostasy. Unfortunately, these words are frequently misunderstood and therefore used inappropriately.

Heresy, as the Catechism of the Catholic Church defines it, is: “the obstinate post-baptismal denial of some truth which must be believed with divine and catholic faith ... .” It is basically believing as true that which is false or as false that which is true regarding the major teachings of our Christian faith.

But for something to be considered heresy, note the requirements. First, it must be “obstinate.” In other words, it’s not merely a mistake or a misunderstanding regarding a belief. Even someone who is very sincere in their faith may at times make a mistake about it simply because of poor catechesis or a lack of understanding.

Heresy, properly speaking, is something that someone freely chooses to hold to despite knowing that their belief is not what is believed by the Church.

Looking at a few historical examples might help to clarify what constitutes heresy: denying that Jesus was truly human, denying that He was truly God, denying the Trinity, or denying the need for God’s grace to be saved. All of these (and many others) are heretical positions.

If a mistaken belief is pointed out to people, and they understand the difference between their belief and the Church’s, and yet persist believing incorrectly, then, and only then, it may well be heresy.

Likewise, as the catechism’s definition points out, it must be “post-baptismal.” One may believe something very different about Jesus than Christians do, but that belief cannot properly be called heresy if it is believed by someone who is not, and never has been, a Christian. Therefore, it would be incorrect to claim that people in non-Christian religions are heretics (properly speaking) because of their beliefs, even if we would say that they are incorrect or mistaken in their beliefs about God, Jesus, etc.

Being baptized brings with it the responsibility to persevere in correct Christian belief, and at the same time to correct in ourselves mistaken beliefs should they ever arise.

Finally, for something to truly be a sin of heresy, it must pertain to something that is to “be believed with divine and catholic faith.” This can be another area that often causes the term to be misapplied.

Heresy really deals with the major beliefs of our faith. Denying a truth found in the Apostles’ or the Niceno-Constantinopolitan Creeds would definitely rise to the level of heresy. As could denying other dogmas or clearly defined and well established teachings of the faith such as the Immaculate Conception or the real presence of Jesus in the Eucharist.

However, disagreeing with other parts of the faith, such as some of the disciplines that the Church holds to (for example requiring celibacy of priests, or the Lenten practice of abstaining from meat on Fridays), while certainly putting us at odds with the mind of the Church, cannot rise to the level of heresy.

Finally, what about apostasy? While heresy denies some truth of the faith, apostasy rejects the Christian faith altogether. It takes a baptized Christian saying “the Christian faith is wrong” or who joins a non-Christian religion to commit the sin of apostasy.

Someone who stops practicing their faith for a time is not yet guilty of it (even if they still have moral responsibility for not practicing).

Both heresy and apostasy remind us of the need to continue learning our faith, so as to grow our appreciation and gratitude for it, and commit to practicing it fully for the whole of our lives.