There is such a thing as believing God and still having your doubts. The fact that a person doubts on a given point, though, doesn’t mean they don’t trust God. People who read the Bible and try to get it right should know this: God is not offended by our doubts.
In fact, it is our willingness to put our trust in him in spite of our doubts that constitutes faith. If there were no doubts, there could be no faith. Faith, in that sense, is trusting God instead of trusting our doubts. And it has to do with the choices we make.
We know that Abraham believed God, and that his faith was counted for him as righteousness. This doesn’t mean, however, that Abraham never doubted. In fact, some of the promises that God made to Abraham could easily be viewed as outrageous. Consider this encounter between the two in Genesis, chapter 17.