I say I know because I know from personal life experiences. I know the difference between belief and knowledge. I also don't expect people who only define knowledge as a provable fact to be persuaded that my knowledge is fact and I don't try to convince them. There is no justification for their insults.

PS: the context of me having said that I know souls exist was it was a comment by me to a writer who claimed that science has proven that souls dont exist. Then another reader is the one who got into it with me. If you want to see the conversation and the difference in the way he and I handled it, it's in my story that I linked