I wrote to you in my letter not to associate with sexually immoral persons not at all meaning the immoral of this world, or the greedy and robbers, or idolaters, since you would then need to go out of the world. (1 Cor 5:9-10 NRSV)
This verse has often been cited by Latter-day Saints as evidence that Paul wrote letters that, while referenced in his epistles, are no longer extant. Some Protestant apologists (e.g, Norman Geisler) has tried to counter this claim by arguing that this is a device called the “epistolary aorist,” and Paul was referencing the letter he was writing at the time (i.e., First Corinthians itself), notwithstanding the vast majority of modern biblical scholarship rejecting this claim (see the discussion by D. Charles Pyle in this paper here). I am currently reading a hefty (1,300+ pages) commentary on this epistle, and the author, commenting on this passage, writes:
Several of the Church Fathers (followed by Erasmus) interpret εγραψα in v. 9 as an epistolary aorist denoting the present act of writing (as in Gal 1:11; Philem 19, 21; and Col 4:8). But this is excluded by ἐν τῇ ἐπιστολῇ, in my [the] letter. The view that this verse makes it certain that Paul wrote "a previous letter" goes back to the early Latin commentator Ambrosiaster, and is endorsed by Calvin, Beza, Estius, Grotius, Bengel, and virtually all modern commentators. (Anthony C. Thiselton, The First Epistle to the Corinthians [The New International Greek Testament Commentary: Grand Rapids, Mich.: Eerdmans, 2000], 408-9; emphasis in original.)
As Thiselton noted, Calvin also held to the view that this letter is no longer extant; in his commentary 1 Cor 5:9, Calvin writes (emphasis added):
The epistle of which he speaks is not at this day extant. Nor is there any doubt that many others are lost. It is enough, however, that those have been preserved to us which the Lord foresaw would suffice. But this passage, in consequence of its obscurity, has been twisted to a variety of interpretations, which I do not think it necessary for me to take up time in setting aside, but will simply bring forward what appears to me to be its true meaning. He reminds the Corinthians of what he had already enjoined upon them—that they should refrain from intercourse with the wicked. For the word rendered to keep company with, means to be on terms of familiarity with any one, and to be in habits of close intimacy with him. Now, his reminding them of this tends to expose their remissness, inasmuch as they had been admonished, and yet had remained inactive.
He adds an exception, that they may the better understand that this refers particularly to those that belong to the Church, as they did not require to be admonished to avoid the society of the world. In short, then, he prohibits the Corinthians from holding intercourse with those who, while professing to be believers, do, nevertheless, live wickedly and to the dishonor of God. "Let all that wish to be reckoned brethren, either live holily and becomingly, or be excommunicated from the society of the pious, and let all the good refrain from intercourse and familiarity with them. It were superfluous to speak as to the openly wicked, for you ought of your own accord to shun them, without any admonition from me." This exception, however, increases the criminality of remissness, inasmuch as they cherished in the bosom of the Church an openly wicked person; for it is more disgraceful to neglect those of your own household than to neglect strangers.
While acknowledging (correctly) Paul is referencing a letter no longer extant in this text, Calvin engages in special pleading by arguing that God did not allow its preservation in his sovereignty as the extant Pauline letters (and rest of the canon) would “suffice” (be formally sufficient). However, outside his ipse dixit, and fallacious reasoning, he and any other Protestants who holds to, not just sola, but tota scriptura, cannot ever be sure of this.
While it is true that just because a volume is referenced in the Bible does not mean that the biblical authors imputed to it the status of God-breathed scripture, can a Protestant claim with 100% confidence this lost epistle was not inspired by God? If they will argue that if it were, God would have preserved it, then what about the book of Deuteronomy that was lost for years until it was rediscovered in 2 Kgs 22? Furthermore, what about "missing books" which are not secular texts (e.g., annals), but said to have been written by prophets? For instance, in 2 Chron 9:29, we read:
Now the rest of the acts of Solomon, from first to last, are they not written in the history of the prophet Nathan, and in the prophecy of Ahijah the Shilonite, and in the visions of the seer Iddo concerning Jeroboam son of Nebat?
All the descriptions of these prophets and their writings reflect the language of divine inspiration, not merely historical works the biblical authors refer the reader to for further secular information, such as "prophet" (נָבִיא); "prophecy" (נְבוּאָה); "visions" (חֲזוֹת) and "seer" (חֹזֶה). Again, only by engaging in special pleading can a Protestant apologist brush off the "missing books in the Bible" argument for these and similar prophetical/apostolic writings.