What does it mean to believe in something? Children are asked it they believe in Santa Claus. We are asked if we believe in God or believe in miracles. In each of these instances, the preposition in is used. The word in can be substituted by the word that. When we say we believe in something we usually mean that we believe that something is true. It is a matter of intellectual assent to a certain proposition. We are saying yes to a certain thesis, making an affirmation about the truth of something.