The Doctrine is Fixed, the Explanation Can Change

The Catholic Church is the guardian of the deposit of the faith, the true teachings handed down to her by Christ. In explaining these truths to people, the Church has through her history made use of the best understandings of the world available through philosophy and natural science.

So, for instance, we know from Christ that in the Eucharist we receive Christ's true Body and Blood. We also know that the bread and wine continue to look and taste like bread and wine. Jesus put a lot of weight on this teaching. Indeed, when many of his disciples leave him over it, he readily lets them walk off. He certainly, doesn't say, "Oh, gee, guys, I was only using a figure of speech!"

Jesus said to them, “Amen, amen, I say to you, unless you eat the flesh of the Son of Man and drink his blood, you do not have life within you. Whoever eats my flesh and drinks my blood has eternal life, and I will raise him on the last day. For my flesh is true food, and my blood is true drink. Whoever eats my flesh and drinks my blood remains in me and I in him. Just as the living Father sent me and I have life because of the Father, so also the one who feeds on me will have life because of me. This is the bread that came down from heaven. Unlike your ancestors who ate and still died, whoever eats this bread will live forever.”

These things he said while teaching in the synagogue in Capernaum.

Then many of his disciples who were listening said, “This saying is hard; who can accept it?”

Since Jesus knew that his disciples were murmuring about this, he said to them, “Does this shock you? What if you were to see the Son of Man ascending to where he was before? It is the spirit that gives life, while the flesh is of no avail. The words I have spoken to you are spirit and life. But there are some of you who do not believe.” Jesus knew from the beginning the ones who would not believe and the one who would betray him. And he said, “For this reason I have told you that no one can come to me unless it is granted him by my Father.”

As a result of this, many [of] his disciples returned to their former way of life and no longer accompanied him.

Jesus then said to the Twelve, “Do you also want to leave?”

Simon Peter answered him, “Master, to whom shall we go? You have the words of eternal life. We have come to believe and are convinced that you are the Holy One of God.” John 6:53-69

And yet, while Christ clearly taught this doctrine, He didn't explain it in the terms of every philosophical system which might come along in the millennia to come. Thus, for instance, in response to questions that arose in the 13th and 14th centuries, the Church defined the real presence of Christ in the Eucharist using the philosophical language of Aristotle, using the term transubstantiation. We've continued to use this term ever since, because Aristotelian philosophy (as used by Aquinas and other Scholastics) happens to be a really good way to explain the real presence: The substance, the true being, the is-ness, of the bread and wine become the body and blood of Christ, and yet their physical characteristics remain those of bread and wine. They look the same, they taste the same. And yet they are truly Christ.

And yet, Aristotelian philosophy is a human system of thought. It is not perfect. There may be ways in which we find that the explanations of the world given by Thomistic-Aristotelian philosophy do not match things which we believe we know from other means. Does this mean that, since many Church doctrines are expressed in Thomistic-Aristotelian terms, that those doctrines may change or simply be wrong due to being based on mistaken philosophical notions?

No.

What we need to remember is that while the Church is responsible for explaining Christ's teachings, the teachings themselves come from Christ. Even if the Church has expressed teachings in terms with which we may later find problems, the teachings themselves are not from the Church and thus cannot be changed by the Church.

So even if we find the Thomistic-Aristotelian ideas of substance and accidence to be problematic in some other areas, and perhaps even come up with other philosophical explanations of the Real Presence, the Real Presence itself is not dependent on the accuracy of the Thomistic-Aristelian understanding of substance and accidence.

Similarly, the doctrine of original sin is not dependent on a Thomistic-Aristotelian understanding of what a species is. Teachings regarding marriage and sexuality, similarly, are not dependent on ancient or medieval human understandings of sexuality or gender for their truth. Etc. If Christ's truth has been explained in human terms that are no longer well understood, then it's important for us to come up with new explanations which address our modern understanding of the world. And yet, it's even more important that we not deceive ourselves into thinking that because we may see some problems with the philosophical system which was used in formulating an explanation, that the doctrine itself is therefore up for grabs or out of date. It isn't. God is eternal and is not subject to the errors of one time or another.