The new covenant — a new way of being in relationship with God replacing the old covenant of externals written on stone — is written on the heart and associated with knowing God. New heart, new spirit, and knowing the sacred go together.
Thus the heart needs to be opened. To use a favorite metaphor, spirituality is "for the hatching of the heart." To extend the metaphor, the heart is like an egg with a shell around it. If what is within is to live, the egg must hatch, the shell must break, the heart must open. If it does not, the life within dies and becomes foul-smelling and sulfuric.
— Marcus J. Borg, The God We Never Knew