An Evil Heart of Unbelief

Why does the Word warn us against unbelief? Do your feelings and emotions have anything to do with belief? Do the regenerated still have hearts of unbelief? How can we know Christ and yet have unbelief? How can we help one another with our struggle every day? Can unbelievers do anything to make themselves believe? What is the fruit of a believing heart? What are we called to examine? Is there a hardness in some that say they know all there is to know about Christ?