“All too often, it is assumed that the unbeliever has made a coherent and rational case for his unbelief. It is assumed that the unbeliever has arrived at his unbelief through genuine intellectual considerations. It is assumed that the Christian and unbelieving positions are on somewhat equal intellectual footing and that the Christian’s job is to show the unbeliever that the Christian position is equally reasonable or perhaps more reasonable than the unbelieving position.”