“To know” for some people (many philosophers included) means absolute certainty, certainty as an emotion (yes, chemicals produced by the brain if you allow the trust of such knowledge). When you make that concept (to know) dependent with such emotion (to “feel” certain), you have already failed: emotions have their own inertia, that is, sometimes is nothing you can do to modify them just because you toss other concepts in their way, let alone ideas (emotions tend to not be influenced by ideas, that’s the whole strength or religions too). “Certainty” can not be invoked by anything but other emotions that have evolved with our species for pragmatic survival reasons.

But “to know” for some other people (including many gnostics) means a given degree of reliability after certain perceptions and/or rational ideas. For those who adhere to this second definition of “to know”, it is enough what is provided by empirical experience and what is (rationally) sound. Lucubrations like “how do you know that we are not slaved inside a giant cosmic computer that juggles with all our perceptions and thoughts” does not invalidate this version of knowledge, but it does invalidate the former one (the one that needs “absolute certainty” to be). The second meaning of “to know” is what has always been adopted by humans when dealing with everyday life: humans just dealt with the pragmatism of their senses and (some degree) of rational thought. (Intellectual and emotional) uncertainty, very useful as a motor for philosophy (because it forces to doubt and reassess each and every concept under a new light, whatever it is) wasn’t a main concern when other stronger experiences were present (generally uncertainty was overridden by palpable facts).

For many theistic people, the “absolute certainty” of the first definition of “to know” is relegated to the dominion of another emotion. It is called “faith”, and it has its own inertia as I said before about emotions. Therefore: When “uncertainty” then (a big chunk of) “faith”. How they extract this second emotion? From our ability as a species to recognize, depend and trust social hierarchies, because we are a social species (we evolved from social animals), thus it is easy for us to “trust” fatherly/chief/strong figures. Philosophers don’t have this emotional advantage: if they use the rational path then “just because there is a strong authoritative figure” is not a rationally valid syllogism (it is a logical fallacy).

You can counter this “theistic gnosticism” (to know for certain after a god) with a “positivistic gnosticism” (to know after trusting empiricism and rational ideas). That is, you have to different gnostic dominions, one whose base is emotional (certainty plus faith) and the other that is perceptional/rational (our senses and reason). From the second one come science and all what provided the civilizations of the world (if you trust your senses right now), from the first one comes subjugation to the religious hierarchy (only god/s gives certainty (therefore legitimacy) to everything that can be known). Atheist agnosticism generally is based on the first definition a mentioned earlier.

“Theistic gnosticism” can say: jump into an abyss, gods commands you to do so (knowledge depends on faith of such superior chief) and rely on his powerful hand to survive. “Positivistic gnosticism” can say: (if you have a well maintained parachute) you can jump into (an already probed) abyss and the higher probabilities are in favor of your survival. (As positivism is based on the recollection of knowledge and this process is still incomplete, probabilities gave an approach to have range of success, the filling the gaps of knowledge doesn’t mean that “everything is possible” as depend on the tested laws of physics, but that’s other theme).

TL;DR: There are mainly two definitions of “to know” what give way to different ways to approach the “gnostic versus agnostic” issue. The theistic “to know” relies in an emotional logical fallacy, the positivist (scientific) “to know” relies on senses and reason; from the second one comes all the achievements of today’s civilization as a shortcut to prove its (pragmatic) approach to what is knowledge.