Why do we exist?
"These things I have spoken to you, that my joy may be in you, and that your joy may be full."--Jesus (cf. John 15:11)
Trying to explain this to someone who has never experienced the true joy of knowing and loving God is very difficult. It is like trying to describe a fillet Mignon steak to someone who has never eaten one. But that is what we try to do. We invite those who don't know God to taste and see. Much of Christian understanding is preceded by experience (though both are necessary for the knowledge of God).