It depends entirely on what is meant by “teaching,” and the Church draws the line sharply. Doctrine — the truths of faith and morals handed down from Christ and the Apostles — she has no power to reverse. Discipline — the practical rules she sets for a season — she changes freely. Fasting laws, whether priests may marry, the language of the liturgy, the days of obligation: all discipline, all adjustable. A defined moral truth is not. The Church claims authority to guard what was revealed and to understand it ever more deeply — never to repeal it.

The reason is not stubbornness; it is the nature of the claim. If a moral truth is true — that the deliberate killing of the innocent is wrong, say — then it was true in the first century and is true now, because truth does not expire with a calendar. The faith was “once delivered to the saints” (Jude 3), and its author is “Jesus Christ, yesterday, and today; and the same for ever” (Hebrews 13:8). A Church that could vote moral truths up or down to suit the age would not be guarding revelation — it would be composing its own.

This does not mean understanding stands frozen. Doctrine genuinely develops — the Church draws out the deposit more fully over centuries, the way she unfolded the full doctrine of the Trinity from Scripture. But St. Vincent of Lérins fixed the iron rule in the fifth century: authentic development advances “in the same doctrine, in the same sense, and in the same meaning”1 — an acorn becoming an oak, never an acorn becoming a horse. Growth that reverses the original meaning is not development at all; it is corruption, and it is exactly what the Church cannot do.

Be honest about the hard cases, because critics raise real ones: the meaning of “no salvation outside the Church” was sharply clarified, her condemnations of slavery grew far more explicit over time, the application of the usury teaching shifted as economies changed. But examine each and you find a development or a disciplinary adjustment — the underlying principle drawn out, not overturned.2 So the answer is precise: the Church can change how she disciplines and how deeply she understands, but she cannot make the sinful holy or the true false. That her core teaching has held for twenty centuries is itself one of the stronger arguments that she is what she claims to be.