"He that will maintain that man's free will is able to do or work anything in spiritual cases, be they never so small, denies Christ." - Martin Luther
Martin Luther hits the nail on the head. Apart from the Holy Spirit, man, when left to himself, cannot rise above what he is by nature, and will use his so-called 'free will' to suppress the truth of Christ. Anyone who thinks they can believe the gospel apart from the Spirit, therefore, denies his need for Christ, not only for justification, but also for the grace needed to be willing to submit to the humbling terms of the gospel.