What does it mean to know God?

What is Christianity? Some say it is a philosophy, others say it is an ethical stance, while still others claim it is an actual experience. None of these things really gets to the heart of the matter, however. Each is something a Christian has, but not one gets to the heart of what a Christian is. Christianity has at its core a relationship between a person and God. A person who becomes a Christian moves from knowing about God distantly to knowing about him directly and intimately. Christianity is knowing God.

Now this is eternal life; that they may know you, the only true God, and Jesus Christ, whom you have sent (John 17:3)