The 1837 edition dropped the conjunction for here in Alma 33:2, probably accidentally. The change was possibly influenced by the preceding if-clause, which does not begin with a for (“if ye suppose that ye cannot worship your God / ye do greatly err”); both if-clauses otherwise have the same initial words (“if ye suppose that”). The for beginning the second if-clause helps connect this if-clause with the preceding discourse. Moreover, the conjunction for introduces if-clauses quite frequently elsewhere in the text (29 times).
Summary: Restore in Alma 33:2 the conjunction for before “if ye suppose that they have taught you this”.