What does this archaic use of "fear never but you" mean exactly?

Some editions of this essay add the word that, making it easier to parse:

Fear never but that you shall be consistent in whatever variety of actions. (source)

Fear never is a synonym for never fear or fear not, so a more understandable rendition might be:

Fear not but that you shall be consistent...

But serves to negate, so if we replace it with a different negation word—say, fail—we get:

Do not fear that you will fail to be consistent...

Other usage examples:

• Time is precious—fear not but that you will be sustained. (source)
  
• Sir Michael, fear not but that you have looked your last on me. I go never to cross your path again. (source)

But in this sense is used as a second negative to cancel an initial negative and so express a positive. The net positive sense here is Rest assured you will be consistent in any variety of actions, provided that each is honest and natural in its hour.

O.E.D. s.v. but, prep., adv., conj., n.³, adj., and pron. Sense 4:

So after a negative, expressed or implied. (Here but regularly translates Latin nisi, and may be explained as ‘unless, if not’. It has been treated as a conjunction from the earliest times.)

Curiously, the two idioms cannot but Verb and cannot help Verb-ing, each by itself the sort of double negative that expresses a positive, have largely merged into cannot help but, which illogically enough is used to express a positive (if I cannot help but laugh, I laugh) with a triple negative.