Here the indefinite article a was accidentally lost before man in the 1858 Wright edition. The 1908 RLDS edition restored the a to the RLDS text. The a is, of course, expected, as in the following cases where “(a) man” serves as the complete subject predicate; in each of these cases, the subject is I:
The first two examples were spoken by Ammon in his attempt to convince king Lamoni that he, Ammon, was not the Great Spirit, thus the appropriateness of a man here in verse 3. The critical text will maintain the a before man in this passage.
Summary: Retain the indefinite article a before man in Alma 18:3 (“whether he be the Great Spirit or a man / we know not”).