What Paul and Joshua reference
In 1 Corinthians 5:9, Paul says he previously wrote to the Corinthians and then explains what he meant. The ordinary reading is that he refers to an earlier letter that has not survived as a separately identified document. The verse gives no evidence that a church included it in its Bible and later removed it. Colossians 4:16 asks the congregation to read a letter from Laodicea. The verse leaves its author and identity open. 1 Corinthians 5:9–11 · Colossians 4:16
Joshua 10:13 and 2 Samuel 1:18 cite the Book of Jasher. These references show that a work by that name was known to the biblical writers. They give us no way to authenticate a later book bearing that title or establish that Jasher was treated as scripture. Robert Bowman’s Protestant response makes a fair distinction between a source a writer used and a book a community received as scripture. The Church’s Bible Dictionary lists Jasher and Paul’s earlier letter under “Lost books.” Joshua 10:13 · 2 Samuel 1:18 · Bowman’s response · Bible Dictionary
What early canon lists show
Eusebius reports that many Christians recognized James, Jude, 2 Peter, and 2 and 3 John, while others disputed them. For Revelation he explicitly reports divided opinion. This tells us how particular books were received. It supplies no account of a council taking those books out of an earlier Bible. Eusebius, Church History 3.25
In AD 367, Athanasius listed the same 27 books found in today’s New Testament. He separately named books such as the Didache and the Shepherd of Hermas that could be read for instruction outside his canon. The two lists show both agreement and disagreement across time. They also show that being read in the Church and being counted as scripture were different things. Athanasius’s letter gives his canon and instructions to his readers. It leaves the reception of that list elsewhere to the historical record. Athanasius, Festal Letter 39, §§5–7
The Council of Nicaea met in AD 325. Its surviving creed and disciplinary canons address other matters and contain no list choosing the books of the Bible. Athanasius’s later letter is a separate document. First Council of Nicaea, creed and canons
What about the Apocrypha?
Tobit and 1 and 2 Maccabees survive and appear in Catholic Bibles. The Westminster Confession places the Apocrypha outside the canon. That is a difference about which preserved books count as scripture. When Joseph Smith asked about the Apocrypha in 1833, Doctrine and Covenants 91 said it contains both true material and human additions and that a further translation was unnecessary. It also says a reader enlightened by the Spirit can benefit. Catholic Catechism, §120 · Westminster Confession 1.2–3 · Doctrine and Covenants 91
What Latter-day Saints claim
The eighth Article of Faith affirms the Bible as God’s word insofar as it is translated correctly. In 1 Nephi 13, Nephi sees plain and precious things and covenants taken away. He also sees later records that make those truths known and establish the truth of the earlier records. The vision gives no list of missing titles or passages. To argue that a particular text was removed, we still need evidence about that text. The Latter-day Saint case for additional scripture rests on the claim that Christ still reveals truth and authorized the Restoration. Articles of Faith 8–9 · 1 Nephi 13:26–29, 39–41 · 2 Nephi 29:7–10
Does a missing letter refute sufficiency?
Luke P. Wilson argues that God preserved the apostolic witness the Church needs and that a claimed later revelation must agree with that witness. Westminster’s sufficiency claim concerns what the Church needs for faith and life. It is compatible with an apostle having sent a letter that no longer survives. The remaining questions are what establishes that this canon is the final rule and what evidence would establish authority for later revelation. Neither question is settled by a missing letter. Wilson’s response · Westminster Confession 1.6 · The Scripture-alone case
A reply you could give
Paul refers to an earlier letter. I’d call that evidence of lost correspondence. To say it was removed from scripture, I’d want evidence it belonged to a canon and was later taken out. I believe God has given scripture again, and I’d make the case for that from the Restoration itself.