Mark 9:23-24 (NKJV)
Jesus said to him, "If you can believe, all things are possible to him who believes." Immediately the father of the child cried out and said with tears, "Lord, I believe; help my unbelief!"
This man said that he believed but then immediately asks the Lord for help with is unbelief. Which is it? In this case, he wants to believe, that is to have faith, but he recognizes that his faith is not perfect that that there are still lingering doubts. So this unbelief is not a refusal to believe but rather recognition of weakness. So he asks for help with his fledging belief. This is a wonderful example of how we should be: willing to believe, willing to trust, but conscience of weakness and totally dependent on the Lord for help.
This is an incredible thought, that the power of God in which "all things are possible" is waiting only on our belief before acting. Lord I believe, help my unbelief!