If the bible were to be taken literally, then one should be able to move a mountain if they had faith the size of a mustard seed (Matthew 17:20). Has any such thing ever happened? Has anyone ever told a mountain to move, and then had it do so? If not, then it must mean that faith doesn't literally work that way, or nobody truly has ever had such faith.
If the verse were true, and people truly believed it, then these "true believers" should be making miracles occur on a daily basis, based on faith alone.