It's very common for Mormons to say "I know ___," as in "I know the church is true," or "I know that Christ is my Savior." Today, we had a man stand up and shamefully declared that he didn't know, but that he believed, and it was one of the most touching expression of faith that I've heard in a good long while.
We also had an investigator sitting near us today, and after the meeting he asked me what people meant when they said that they knew something that is generally considered to be unknowable. I told him that it was a way in our culture of expressing that one has had a spiritual experience confirming the belief in question, but that often times we just say I know instead of I believe out of cultural habit. People simply like the strength implied by knowledge.
I'm somewhat comfortable with this specialized cultural context of knowledge, but my main struggle with this phrasing is that it discourages people that don't know, or that haven't had confirming spiritual experiences. It's exclusive: you're not in the club because you don't know. Because of this, the use broadens to cover simple belief as well, making the club more inclusive, but making things even more painful for those who feel they cannot honestly declare knowledge when they have none.