For forty days and forty nights heavy rain poured down on the earth. - Genesis 7:11-12

While the number "forty" may reference well to the same number to other parts of the Bible, I don't see what's so allegorical about "In the six hundredth year of Noah’s life, in the second month, on the seventeenth day of the month". It looks historical to me.

Is there any place where it's declared dogmatically that the beginning must have been a finite time ago? In other words, must Catholics believe that "beginning" is referring to a moment in space and time?

I find your references helpful but inconclusive on this point.

Many thanks for the education!

You have not grasped the Church's position on this.

The official and infallible Church teaching easiest for me to give you is Genesis 1:1; in principio creavit Deus cælum et terram.

From canon 1.5 of Vatican I: " If anyone does not confess that the world and all things which are contained in it, both spiritual and material, were produced, according to their whole substance, out of nothing by God... let him be anathema." A beginning to the world in time is clearly indicated.

It's deducible from this, by anyone who recognizes the Church's authority, that science will never and logically can never prove that the universe had no beginning. This is precisely because there is one truth and reason and faith cannot conflict. There is no risk at all, not to faith and not to rationality, because logical contradictions are impossible.

Even if we did not have that canon from Vatican I or Genesis 1:1 or any other formal declaration - that the world began has always been a constant assumption behind the words of the Fathers and the Magisterium and the Scriptures (for which reason the Church in its ordinary magisterium has continued to teach that the world began), and that alone would be enough.

Then you open yourself up to becoming the newest generation of flat-earthers, geocentrists and special creationists; in other words, pseudoscientific religionists.

Now, maybe you won't. Best scientific evidence currently is that the universe had a definite beginning, but there's no evidence currently one way or the other about whether it came from nothing. It is conceivable, however unlikely, that future work in science will discover that the cosmos never had a beginning, or started but not from nothing.

My money is that you can hold to your (in my opinion entirely unfounded and speculative) dogmatic belief about creation ex nihilo in space and time, and will safely avoid becoming a pseudoscientific religionist. But it's a risk. That's all I was saying. You risk either your faith or your rationality.

You asked about the Catholic Church, not individual fallible Catholics, let alone non-Catholics. Vatican I defines the matter clearly for all Catholics. If someone somehow does not think that the productio ex nihilo spoken of in canon 1.5 of Vat. I implies an act before which nothing existed, then the witness of Genesis 1:1 (with many other passages), the unanimous Fathers, and the constant teaching of the Church is more than enough to leave the answer totally without doubt. This belief has been foundational to Christianity so long it is inherited from the Jews.

Because some Christians don't think that the universe necessarily had a beginning. Some of the Catholics who replied to my comment don't think that the universe necessarily had a beginning.

Science will never show that the universe had no beginning. The Church's teaching is certain on this point. Faith and reason will not contradict, and the teaching of the faith proves that the universe had a beginning.

If the Faith were false, and science did prove that the universe had no beginning, all Christians would be up a creek. I don't understand why you think Thomists in particular would be.