The word "heart" appears well over a thousand times in the Bible. Most often, it is a comprehensive metaphor for the self. It covers much more than does the metaphorical meaning of "heart" in contemporary English. In our usage, the heart is most commonly associates with love, as in Valentine hearts; courage; as in brave hearts; and grief, as in broken hearts. But in the Bible, the 'heart' includes these and more: it is a metaphor for the inner self as a whole. . . . The heart is an image for the self at a deep level, deeper than our perception, intellect, emotion, and volition. As the spiritual center of the total self, it affects all of these: our sight, thought, feelings, and will. . . .
The condition of the heart matters. The heart, the self at its deepest level, can be turned toward God or away from God, open to God or closed to God. But its typical condition is that it is turned away from God and "closed." . . .
When our hearts are closed, we live within a shell. To extend the egg metaphor: the shell needs to be broken open if the life within it is to enter into full life. What we need is the 'hatching of the heart' - the opening of the self to God, the sacred - is a comprehensive image for the individual dimension of the Christian life.
— Marcus Borg, The Heart of Christianity