The printer’s manuscript reads “of Messiah”, as if Messiah is a personal name. The 1830 typesetter emended the text by inserting the definite article the before Messiah, thus interpreting Messiah as a title. The 1874 RLDS edition removed of the, thus changing coming from a gerund to a participle (“the coming Messiah”).
Elsewhere in the Book of Mormon text, Messiah is always treated as a title rather than as a personal name. In fact, in all other cases, Messiah is preceded by a determiner:
“the Messiah” 17 times
“the true Messiah” 3 times
“a Messiah” 3 times
“the holy Messiah” 2 times
“a false Messiah” 1 time
“another Messiah” 1 time
“one Messiah” 1 time
“this Messiah” 1 time
“that Messiah” 1 time
Included in this list are two more occurrences of the phrase “the coming of the Messiah” and one of “the coming of a Messiah”:
On the other hand, there are no occurrences of Messiah preceded by a present participle (such as coming). The 1874 RLDS change from “the coming of the Messiah” to “the coming Messiah” was undoubtedly a typo. The 1908 RLDS edition restored the 1830 reading, not the reading in 𝓟.
Given the otherwise systematic use of Messiah as a title rather than as a name in the Book of Mormon, the 1830 typesetter’s emendation seems quite reasonable. One other possibility, also quite reasonable, is that the original text read “the coming of a Messiah” (as in 1 Nephi 1:19). There are two other uses of “a Messiah” in the Book of Mormon text:
Given the usage elsewhere in the text, either the definite article the or the indefinite article a could have been accidentally omitted here in Helaman 8:13. And we have examples of scribal diffi- culty with both these articles, as in the following cases where there is a preceding of and Oliver Cowdery was the scribe:
of the > of
Statistically, Oliver Cowdery’s deletion of the after of is more frequent than his deletion of a after of. Nonetheless, the loss of a is possible in this context.
David Calabro points out (personal communication) that these arguments depend on the use of Messiah as a title within the text of the Book of Mormon. In contrast, the King James Bible uses Messiah (and its New Testament variant Messias) as a name, with the result that in most of the biblical cases there is no article at all, especially in the original Hebrew and Greek:
These biblical examples argue that Messiah can lack a determiner and thus provide support here in Helaman 8:13 for the reading in 𝓟, “the coming of Messiah”.
Calabro also points out that we can find evidence for no determiner in the words that John the Baptist spoke on 15 May 1829 when he ordained Joseph Smith and Oliver Cowdery to the Aaronic Priesthood. Although the textual sources for this language date from the mid to late 1830s, they consistently avoid the definite article the before Messiah:
There is also an example in his editing for the 1837 edition of the Book of Mormon where Joseph Smith apparently used Messiah as a name, at least initially, when he decided to replace the first instance of the name Jesus Christ in the small plates of Nephi with Messiah:
Joseph spelled the name Messiah as Mosiah in 𝓟, probably because he pronounced both names identically. But the 1837 edition ended up supplying the definite article, thus “and the Messiah”, which has been the reading in all subsequent editions of the Book of Mormon. (For a complete discussion of this emendation of the text, see under 1 Nephi 12:18.)
In both these examples, one from church history and the other from Joseph Smith’s later editing of the printer’s manuscript of the Book of Mormon, Messiah is treated as a name. In fact, in John the Baptist’s phrase “in the name of Messiah”, the word name is explicitly provided. Similarly, the Bible uses Messiah and Messias as the name for the Anointed One. So if we exclude the case of Messiah here in Helaman 8:13, there is a systematic difference: the Book of Mormon uses Messiah as a title (thus supplying a determiner) while the Bible and other scriptural sources use it as a name (thus generally avoiding the determiner).
Given the occurrence of the Messiah in Mosiah 13:33 (which, like Helaman 8:13, refers to Moses’s prophesy of “the coming of the Messiah”), I would favor the definite article the in Helaman 8:13. Moreover, only in the small plates do we have prophets referring to “a Messiah” (Lehi in 1 Nephi 1:19 and 1 Nephi 10:4, and Nephi in 2 Nephi 25:18). Thus the 1830 typesetter’s emendation to the Messiah in Helaman 8:13 is consistent with usage elsewhere in the text and most likely represents the original reading.
Summary: Accept in Helaman 8:13 the 1830 typesetter’s insertion of the before Messiah (“the coming of the Messiah”); Messiah is otherwise used as a title in the Book of Mormon, and Moses is cited as referring to “the coming of the Messiah” in Mosiah 13:33; only in the small plates of Nephi do Lehi and Nephi refer to “a Messiah”; Messiah is used as a name in other scriptures, but not (it would appear) in the original text of the Book of Mormon.