Believe in is a common phrase used to express one's trust, faith, and confidence in something or someone. It might be related to one's personal beliefs, religion, or even an idea or a concept. However, there are many other ways to express one's conviction or trust besides using the phrase "believe in." Among the possible synonyms are "have faith in," "trust," "rely upon," "count on," "depend on," "bank on," "have confidence in," "have conviction about," "be convinced of," and "place one's trust in." These synonyms might also reflect a slightly different shade of meaning or connotation, emphasizing different aspects of trust or confidence depending on the context.