Q: It seems to me that the term “believe” means something more than just to believe in one's mind. Could you explain?

A: You are right; believing is more than just believing in one's mind. The basic meaning of the word in Greek is believing in the sense of trusting. The point here is that we trust Yeshua's work on the cross alone for our salvation and nothing else. Anyone who trusts anything else for salvation is believing a false gospel.