Here in Alma 45:19, the 1879 LDS edition changed the singular scripture to the plural scriptures, probably accidentally since the resulting “scriptures saith” is technically ungrammatical: saith is supposed to take a third person singular subject, not the plural scriptures. But this argument has no force at all since verb forms ending in -(e)th frequently took plural subjects in the original text of the Book of Mormon. For the 1879 edition, one might expect the editor, Orson Pratt, to have been aware of the distinction. Perhaps the error was introduced by the 1879 typesetter.
The singular scripture is used a number of times in the Book of Mormon to refer to a single scriptural passage; in each case, the verb is say (in fact, the verb form is consistently saith). In the following four cases, I list the King James passage that most clearly matches the language of the corresponding Book of Mormon passage:
The example in Mormon 8:20 shows a tendency on the part of scribe 2 of 𝓟 to initially write the plural scriptures; in that instance, he immediately corrected the plural to the singular by erasing the final s. The 1830 edition, which is a firsthand copy of 𝓞 for this part of the text, also reads scripture. We can be quite sure that 𝓞 itself read in the singular for that passage.
It is worth noting that the two New Testament scriptures (referred to in Mormon 8:20 and in Ether 13:12) may each derive from some earlier, now unknown, scriptural source. In the case of Mormon 8:20, Paul’s reference in Romans 12:19 to what is written may be a paraphrase of Deuteronomy 32:35: “to me belongeth vengeance and recompense”. There is also the possibility that the specific language of Romans 12:19 and Mormon 8:20 actually derives from some earlier scriptural passage, now lost, that was related to the Deuteronomy passage. Similarly, David Calabro points out (personal communication) that Moroni’s citation in Ether 13:12 may be a paraphrase of the earlier Nephite scripture in 1 Nephi 13:42: “and the last shall be first and the first shall be last”. One could also argue that the specific language of Luke 13:30 and Ether 13:12 originated with some earlier, now unknown, scriptural source.
Here in Alma 45:19, the reference is to Moses’s passing; this event is described in the following biblical passage:
This passage states that the Lord buried Moses, which agrees with the statement in Alma 45:19 that some thought Alma had been “buried by the hand of the Lord / even as Moses”. But the phrase “even as Moses” may apply to the entire preceding conjunctive phrase “he was taken up by the Spirit or buried by hand of the Lord”; note that immediately afterwards the Book of Mormon writer (either Helaman or Mormon) tacitly assumes that Moses and Alma were received “in the spirit” (especially since the text uses the word also):
But the writer in Alma 45:19 refuses to speculate on whether Alma was physically “taken up by the Spirit” in a literal sense or even buried by the Lord. The possibility remains that the Book of Mormon writer is referring to a specific but unknown scripture that says, as Alma 45:19 does, that “the Lord took Moses unto himself ”.
The original Book of Mormon text also has one case where the plural scriptures occurs with the verb say (and once more with the verb form saith, at least originally); in that instance, Christ is speaking to his twelve Nephite disciples:
Here the original manuscript is not extant, but both the 1830 edition and the printer’s manuscript have the plural scriptures. Since both are firsthand copies of 𝓞 for this portion of the text (from Helaman 13 through the end of Mormon), we can assume that 𝓞 also read scriptures for this passage. However, the example in Mormon 8:20 shows that the scribe could have accidentally written the plural scriptures in 𝓞. Moreover, as explained earlier in this section, the fact that the earliest text has the form saith cannot be used as an argument against having a plural subject such as scriptures.
It turns out that this passage in 3 Nephi 27:5 is conceptually different from the five others that read “the scripture saith”. There is no scriptural reference in the Bible that refers to “taking upon oneself the name of Christ”, but there are numerous references in the Nephite scriptures that use this phraseology:
Christ, the speaker in 3 Nephi 27:5, is thoroughly familiar with the Nephite scriptures; see 3 Nephi 23:6–13, in which Christ emphasizes the importance of Samuel’s prophecy about the resurrection being included in the Nephite scriptures. Also note that 3 Nephi 23:14 refers to Jesus expounding to the Nephites “all the scriptures in one which they had written”. Consequently, there is no reason that Christ would necessarily restrict himself in 3 Nephi 27:5 to referring to just biblical scriptures. Thus the use of the plural scriptures in 3 Nephi 27:5 is perfectly appropriate.
Summary: Restore the original singular scripture in Alma 45:19; elsewhere the text uses “scripture saith” whenever a single scriptural passage is referred to; on the other hand, the plural scriptures in 3 Nephi 27:5 is correct since in that case Christ is referring to the multiple passages in Nephite scripture where prophets have exhorted the people to take upon themselves the name of Christ.