To believe God involves believing doctrine, but it involves more than that.  It is actually believing Him.  There are many who believe sound doctrine, but do not believe God.  The foundation of their faith is not that God said it, but that their parents or their pastor said it.  One can go so far as to believe sound doctrine because the Bible says it, yet not believe God, for so did the Pharisees in Christ's day.  To believe God is to believe His word because it is He that said it.  Let me illustrate:  Suppose a man came to you saying, "I have a sure-fire investment opportunity.  Whatever you invest, I guaranty I can double it in one year."  Of course, you are skeptical.  But then, an old friend walks up saying, "Oh, I see that you are talking to so and so about investments.  I tell you, he's a sure-fire broker."  So you decide to invest with the man.  Now, whom did you believe?  You did not believe the broker, but your friend.  Now you see what I mean when I say that believing God is believing His word because it is He that said it.  Do you believe God?