Here the 1953 RLDS edition replaced this with his in “this holy calling”; this change is probably a typo, perhaps facilitated by the use of his elsewhere in this passage. The use of this here is undoubtedly correct; note its use in the two preceding present participial clauses:
The critical text will therefore follow the earliest reading, “this holy calling”, in Alma 13:6.
Summary: Maintain in Alma 13:6 the use of this in “this holy calling” (the earliest extant reading) which is consistent with nearby usage in verses 4 and 5.