Faith is believing what you know ain’t so. – Mark Twain
Faith is not merely believing something, it is specifically believing something without evidence.
A belief is a claim that is accepted as true, which then becomes a rule of action. The word belief does not specify on which basis someone believes it.
For religious people, faith is “the evidence of things not seen” (Hebrews 11.1 KJV). That is, faith is a belief that has no evidence for it. Faith takes the place of evidence. “Faith is the evidence.”
To be a little clearer, it is faith in the authority of a book, prophet, or guru. So faith rests on the logical fallacy known as the appeal to false authority.
A prophet, apostle, or some religious leader is not an authority on the nature of reality. And every religion contradicts another religion. So they can’t all be true.
So Mark Twain is not that far off when he says, “Faith is believing what you know ain’t so.” Because knowledge is based on evidence, faith is not.
Faith is believing something without evidence. It is, in other words, accepting as true what you have no good reasons to believe is true.