When you give your soul to a person, thinking they will never hurt it. Trust is when you believe with your entire heart that a person will not turn you around and stab you in the back.
Knowing that no matter what happens, that a person will always be there for you. The most important thing in a relationship. Something worth fighting for. Our greatest power and weakness.
Treating somebody as a friend doesn't mean you trust them, it means you don't think you're better than them, and you don't think they're better than you.