The use of art here is nonstandard for Early Modern English. The antecedent for the relative pronoun which is the third person singular noun phrase a holy man, which means that from a grammatical point of view the verb form should be is rather than art. Joseph Smith, in his editing for the 1837 edition, replaced the art with is. (Of course, the archaic use of which for a person was changed to who for the 1837 edition. For discussion of that change, see under WHICH in volume 3.)
One could view the use of art here as simply an error based on the earlier thou in the passage (“for thou shalt feed a prophet of the Lord”). In several other places in the original text, nonstandard use of art is found in a relative clause. In each case, the antecedent for the relative pronoun itself is either a first or third person singular pronoun:
Of these other cases, only in Alma 36:18 is there a preceding second person pronoun that could have prompted the use of the art (“O Jesus thou Son of God”). Thus the nonstandard use of art cannot be explained simply as an error based on the occurrence of a preceding second person singular pronoun form (such as thou). The critical text will restore the use of art in all these cases since its use is restricted to a specific context (a relative clause) and appears to be fully intended. There are two other instances where such a nonstandard art is found in the earliest text—namely, in Mosiah 2:21 and Alma 5:37. In those two instances, art begins a predicate that is conjoined by and to a preceding predicate that begins with hath (for discussion, see under Mosiah 2:21).
Summary: Restore the nonstandard use of art in Alma 10:7 (and elsewhere in the text in relative clauses where the earliest textual sources support its occurrence).