There is some question concerning what the word witness means here. One possibility is that witness refers to another person (or to other persons), besides Jesus himself, who will witness to the Jews that Jesus was the Christ. Joseph Smith’s editing for the 1837 edition of that which to him whom supports this interpretation.
Another possibility is that witness refers to the testimony itself rather than the person witnessing— for instance, when we refer to someone’s witness or to the witness of the scriptures or to some other kind of evidence. Elsewhere in the text, we have examples of witness(es) that refer to witnesses as people and others that refer to testimony itself. But when the noun witness(es) occurs without the article a /an or the number one, the word refers to testimony:
This usage suggests that other witness in Mormon 3:21 refers to testimony rather than to a person, in which case the change from that which to him whom would not work. Of course, the critical text will maintain the original that which even if it does refer to people.
In fact, it is theoretically possible that the word witness, no matter whether it refers to persons or testimonies, stands for the plural witnesses; thus the text may be saying that “the Jews … shall have other witnesses ... that Jesus whom they slew was the very Christ and the very God”. But as Don Brugger points out (personal communication), here in Mormon 3:20–21 it appears that Mormon is referring to his own writing (or the Book of Mormon more generally) as the other witness of Jesus as the Christ. Yet even if the plural is the correct way to interpret witness, the critical text would maintain the bare form without the -es ending. For a discussion of the possibility that witness sometimes stands for witnesses in the original text, see under 2 Nephi 31:18.
Another possibility here in Mormon 3:21 is that other witness is an error for another witness. Of course, all these alternative readings involve some conjectural emendation, whereas the earliest text with witness does work (“the Jews … shall have other witness besides that which they saw and heard”). Don Brugger (personal communication) has provided the following example of the usage “have other witness” from :
Note that in this citation the noun phrases other witness and such witness are singulars since the associated verb is the third person singular makes. The critical text will maintain the earliest reading in Mormon 3:21 and assume that witness is in the singular.
There is one other variant here in Mormon 3:21, namely, when the 1906 LDS edition replaced besides with beside. But that edition never served as a copytext, so the incorrect beside was never transmitted to any subsequent edition. As explained under Alma 57:6, the original Book of Mormon text does not use the adverbial form beside, only the form ending in s, of which there are three instances, all meaning ‘in addition to’.
Here in the 1906 edition for Mormon 3:21, the change was from “besides him whom” to “beside him whom”; that is, the grammatical change from that which to him whom had already occurred (in the 1837 edition). Interestingly, one of the uses of besides in the original text is like the current text in Mormon 3:21:
The form beside, it turns out, does occur in biblically styled language. Note especially that the expression “beside that which” occurs four times in the King James Bible:
Also note that no biblical instance of “beside that which” refers to a human. Thus these biblical examples, except that they read beside instead of besides, confirm that in Mormon 3:21 the expression “besides that which” is used with witness to refer to testimony and not to a person.
Summary: Maintain in Mormon 3:21 the singular form witness; restore the original that which since that is the reading of the earliest text; most likely, the word witness is referring to testimony rather than to people, which means that the 1837 change to him whom is not possible; also maintain the form besides instead of the biblically styled beside.