One of the most common arguments against Christianity is that the God of the Old Testament seemed so drastically different than the God of the New Testament. In the Old, many claim He seemed cruel and capricious, while in the New, He suddenly became a God of love and tolerance. Yet Jesus said, "If you've seen Me, you've seen the Father" (John 14:9), and the writer of Hebrews tells us that "Jesus Christ is the same yesterday, today, and forever" (Hebrews 13:8). If God hasn't changed, how do we explain the apparent difference? The answer lies in a long-forgotten word: covenant.

Covenants play little to no role in our world today, but in ancient times they were all-important. Treaties between individuals, tribes, and kingdoms took the form of blood covenants. These were much stronger than our modern concept of a contract, which can be broken by finding clever legal loopholes or sometimes simply by a decision not to honor one's word. By contrast, covenants were bonds broken...