I think people only can "know" their own experiences.
This is why I am comfortable with the following statements from anyone, regardless of religious affiliation, among others:
"I know that God lives and loves me."
"I know that our situations and circumstances are known to and understood by God."
"I know that there is great joy and peace that comes from striving to acquire the characteristics of godliness found in our scriptures."
"I know we are blessed when we serve others - especially when we do so with no expectation of reward other than what comes into our hearts."
There are lots of things I "believe" - but there also are lots of things that my own, personal experiences have taught me in ways that are undeniable to me. I'm fine saying I "know" these things - especially since I agree we "know" (for ourselves) those things we experience and internalize.
Finally, since I reserve the right to define terms according to what makes sense for me, I have NO problem with others doing the same - even when how they define things differs from how I define them. As I've said here previously, if I want that consideration from others, I really should be willing to grant them the same consideration. If someone feels comfortable saying they "know" certain things (almost always based on their own experiences), who am I to say they are wrong? I don't want to carry the burden of judging others that way, so I don't.
Of course, that means I and someone else can "know" opposite things - but that's ok. Chances are, neither of us is 100% correct - so what does it hurt me to be charitable in this way? Nothing.