I would never die for my beliefs, because I might be wrong. -- Bertrand RussellIt seems simple common sense that a person’s belief is no guarantee of its truth. Nor does that person’s disbelief by itself establish any falsehood.
However, we use the verb “believe” in an interesting variety of ways. It need not merely indicate some person’s state of mind relative to a possible fact or falsehood, but in many contexts morphs into an expression of confidence; or, even more strongly, a declaration of commitment -- often called “faith.”
(In some parts of our multicultural USA, prefacing a statement with "I believe that..." serves as a warning that a somewhat "Sacred Value" is being expressed and is not to be challenged. See Thumper's mother on this etiquette at Goodspin)
Compare this first set of expressions, (call it group A):
a. John is out taking a walkConsider, now, the following group B
b. John will keep his promise to us.
c. The God of the Bible exists
a. I believe that John is out taking a walk.Why use these longer forms? Prefacing a sentence with “I believe that…” seems to be hedging our bets. We are already somewhat on the defensive, since merely asserting something from group A would normally be understood to be something we believe.
b. I believe that John will keep his promise to us.
c. I believe that the God of the Bible exists.
Now, the group B expressions vary substantially in their relation to the following disclaimer: “…but I might be wrong.” They range from expressing almost throw-away facts (So what if I’m mistaken. Big deal!) to possibilities of disappointment (I hope John doesn’t let us down!) They can go even further into the area of deeply held convictions, where it is difficult for many to imagine adding Russell’s complement, “… but I might be wrong.”
For example, take a statement held by some community of believers (whether theistic, philosophical or secular) to be fundamental. Let FB symbolize some fundamental belief. Anyone who would say “I believe that FB, but I might be wrong,” would not likely be perceived by that community as a member, especially if community members expressed their belief as “I believe in FB." (Even such as Credo quia absurdum est.) Just as Hope tends to deprecate Experience, so Fidelity does Truth.
“Believe in” indicates a commitment to a presumed consensus, a trust. Such commitments are not evaluated in terms of their clarity, or their truth, but in terms of their fervor, and the strength of the bonds of allegiance they are believed to generate to a community of consensus.
For references and to examine these issues further, see The Indeterminacy of Consensus: masking ambiguity and vagueness in decision