The concept "heart" describes the deepest, most personally "real" part of ourselves. A great many people have never revealed this Self to anyone, not even a spouse or other family member. At the same time, any hope another person has for truly knowing you depends entirely on getting to "the heart of the matter" within that relationship. This is one of the great truths that Jesus came to share, and it rests at the core of what Christians mean by "having a relationship" with God.
Different Drummer: Frederick Buechner