Jeremiah 17:10

Even though we cannot understand our hearts, the Lord searches them and knows our inner thoughts and motives. "Heart" and "mind" (lit. kidneys, from the Heb. kelayoth, meaning "hidden depths") are not that distinct in Old Testament psychology; they are virtually synonymous here and in many other places. Together these terms cover the range of hidden elements in human character and personality.

God gives to each person what he or she actually deserves. He judges on the basis of works because what we do reflects what we truly value, the condition of our hearts.