To believe in Christ is to give your heart to Christ, which means not to affirm things about Christ, but it's like what you mean when you say, "I believe in my friend."
Frederick BuechnerTo believe is not intellectual assent: "Yes, I believe in Jesus. I will sign my name to the Nicene Creed. I believe it all" - which you could do, [but] it would have no effect on who you were or what you did. It is, rather, to give your heart.
Frederick Buechner