Does Jude 3 close the canon?

I take it seriously. Jude calls the faith already given the standard to defend. Verse 4 shows the danger he has in mind when he says that.

You honor the force of the phrase. Jude is addressing people who distort grace and deny Christ.

I think it just means the first time, so nothing about the faith has to stay fixed.

Jude’s point is to defend the faith already received. Treat that faith as a standard before deciding what the phrase says about later writings.

Giving the faith and giving a further witness to it are separate acts. Jude writes to defend a faith his readers already had. What would rule out God authorizing a later witness?

You identify the extra premise. The analogy shows how a new writing can preserve an existing faith. The question of who can authoritatively speak after the apostles still needs evidence.

Jude gives no list of books, so he proves that any later prophet could add to the Bible.

The lack of a list limits the conclusion from Jude. Authorization for a later prophet is a separate claim that has to be examined.

I understand you’re drawing on Scripture and Tradition about public revelation. I also believe Christ’s saving work is complete. Can we look at what supports the further claim that He would authorize no new scripture?

You describe the Catholic claim fairly and identify the point of disagreement. Catholicism distinguishes new public revelation from private guidance.

Catholics believe God never speaks to anybody anymore, and Jude never says that.

The Catholic Catechism allows for private revelations that help people live the faith. Address its actual claim about public revelation.

Start with a specific claim. Paul and the Book of Mormon both put Christ’s saving work at the center. If you think a particular teaching about grace or ordinances changes the gospel, let’s compare the passages.

You give a concrete comparison and ask to test the part that remains disputed. Agreement on Christ’s saving work does not settle every other doctrinal question.

Both books talk about Jesus, so we can assume they agree on everything that matters.

A shared subject is only a starting point. Compare what each text actually teaches about the issue your friend named.