The original text here (“if ye deny again that there is a God and also deny the Christ”) is an example of a Hebrew-like conditional clause that has the force of an oath and is equivalent to a negative imperative. The construction here basically means ‘do not deny again that there is a God and [do not] also deny the Christ!’ (with the implied meaning ‘otherwise something bad will happen’). As Sommer Greer points out (personal communication, September 2003), this kind of usage is found in English discourse when speakers use incomplete if-clauses without any drop in intonation (as in “if you do that … ”); such if- clauses are interpreted as negative imperatives (meaning ‘don’t do that!’).
In his editing for the 1837 edition, Joseph Smith removed this literal Hebrew-like construction by changing the clause into an interrogative: “Will ye deny again that there is a God and also deny the Christ?” (he even added a question mark in 𝓟). Like the original if- clause, this yes-no question can also be given a threatening interpretation. The critical text will restore the original reading here. In a printed text, for punctuation one could perhaps use three dots of ellipsis at the end of the if- clause as a mark of incompleteness:
For a complete discussion of this Hebrew-like usage, see under 1 Nephi 19:20–21. In that passage the original text provided an example of an incomplete conditional clause that in context functioned as an emphatic declarative. Also see the discussion under hebraisms in volume 3.
Summary: Restore the original if in Alma 30:39: “if ye deny again that there is a God and also deny the Christ”; this Hebrew-like conditional clause acts as a negative imperative in this context.