Non nisi


Can someone help me make sense of "non nisi" in this sentence?

"Ac proinde etiam electi antequam credant, imo cum Evangelio sese adhuc hostiliter & acerrime opponunt, nihilominus divinae electionis beneficio, dignissimi sint, ac proinde veste nuptiali non nisi dicis causa opus habeant, ut digni fiant."

And thus those chosen before they believe, even while they fiercely and aggressively oppose the Gospel; nevertheless, thanks to divine election, they are most worthy, and thus they have no need for a wedding garment unless you say that the reason is so that they be made worthy.

Is "non nisi" supposed to be "nonnisi" and therefore translated together as "not unless," or is my rendering more correct?