True fear of sin “should be a constant thing” Ramchal suggests. That’s to say that “you should always be afraid of stumbling and doing something or some half of something that countervails the glory of G-d”.
Essentially, you should always be anxious to one degree or another that some “sin might enter into or mix with your actions due to some negligence or weakness” on your part, “or for some other unconscious reason”. The point is that that possibility should matter to you every bit as much as slipping up in a performance or even in a rehearsal would matter to a master musician.
In other words, you should so adore G-d and be so enamored of His presence in the world that nothing would threaten or undo your sense of what’s right more than you yourself playing a role — however minor — in its tarnishing.
In fact, “even when you don’t see a stumbling-block before your eyes” that would trip you and have you fall into sin, “you should (nonetheless) be concerned that one might be hidden” from you, “and that (in fact) you aren’t being careful enough” even if you really are careful.
Here then “is the sort of fear a person should have” at bottom in order to achieve spiritual excellence, as Ramchal lays it out. One should “always be agitated” about the possibility of sinning, “and should never allow this fear to leave him”.
Follow through on that, he adds, and “you’ll never come to (purposefully) transgress”. And if you do somehow sin inadvertently, then the sin “will be considered to have been done under duress”, and you’d have been credited with doing everything you could to have avoided it.