Take heed, brethren, lest there be in any of you an evil heart of unbelief, in departing from the living God. Hebrews 3:12
We think of the heart as the seat of the emotions, but the Hebrews thought of it as the core of a person—emotions, intellect, and will. The heart is the wellspring of motivation. When the author speaks of the heart believing something, he is talking about deep convictions held in the core of one’s being, the beliefs that really determine what one does.
Likewise, to harden one’s heart is to make one’s will, intellect, and emotions all insensitive to God’s presence and truth.
(NavPress Bible study, Hebrews, Colorado Springs, CO: NavPress, 1989, pg 54)