It’s not unusual for people to read the Bible with big questions. One of the most common is how to reconcile what seems to be a God of wrath in the Old Testament from what seems to be a God of mercy and grace in the New Testament.

My friend, Tom, is a good example. He became a Christian in his late teens and began reading the Bible. Those who shared the gospel with him repeatedly told him that “God is love.” He believed them, and it was a primary reason for his conversion. But as he read the Old Testament he watched an angry God obliterating humanity in a flood, incinerating Sodom and Gomorrah, and commanding the Angel of Death to kill all the firstborn sons in ancient Egypt.

When he came to the New Testament, however, he encountered a seemingly compassionate and gracious Jesus who obviously liked people, mercifully overlooked their failings, healed the sick, raised the dead, and fed the crowds. When a theological-student friend reminded him that God never changes he was even more confused. When he came to me he found a friend who understood. I struggled with the same issue in the years after my conversion. Because the solution amplified my spiritual joy, I couldn’t wait to share it with Tom.

The solution rests on two basic theological presuppositions. The first Tom already understood: God never changes. “Jesus Christ is the same yesterday and today and forever” (Hebrews 13:8). God’s anger toward sin and sinners is always the same, and it is to be feared. God’s “steadfast love,” repeatedly emphasized in the Old Testament, also never changes, and it is to be enjoyed.

The second principle is the cross of Christ. Augustine famously wrote that the Old Testament in the New Testament is revealed just as the New Testament in the Old Testament is concealed. In other words, we understand the Old Testament by reading the New Testament. Nowhere is this truer than with the cross of Christ. When Jesus agreed to bear our sins in his person, God poured out his wrath on his Son. He punished him for my sins and yours. And the punishment was horrific. Without a doubt, crucifixion was the most brutal and barbaric form of capital punishment ever developed. God displayed the depth of his anger toward both sin and sinners at the cross.

In other words, the cross confirms the truth of the Old Testament prophet, Nahum. “The Lord is a jealous and avenging God; the Lord is avenging and wrathful; the Lord takes vengeance on his adversaries and keeps wrath for his enemies. The Lord is slow to anger and great in power, and the Lord will by no means clear the guilty. His way is in whirlwind and storm, and the clouds are the dust of his feet” (Nahum 1:2–3). As Jesus hung on the cross God avenged himself on his Son for our sins. In the process, he confirmed the words of Nahum.

“Both God’s love and God’s wrath are ratcheted up in the move from the old covenant to the new, from the Old Testament to the New. These themes barrel along through redemptive history, unresolved, until they come to a resounding climax—the cross. Do you wish to see God’s love? Look at the cross.”

D. A. Carson, The Difficult Doctrine Of The Love of God, pg. 70

But the cross is also the most potent display of God’s love in the Bible, for there God proved the truth of 1 John, “God is love.” And this love is more than affection. It is action. It is our happiness at God’s infinite expense. Astounding love motivated God the Father to send his Son to die for unworthy enemies. The Old Testament sums it up in Psalm 103: 11-13. “For as high as the heavens are above the earth, so great is his steadfast love toward those who fear him; as far as the east is from the west, so far does he remove our transgressions from us. As a father shows compassion to his children, so the Lord shows compassion to those who fear him.”

As I shared all of this with Tom we both rejoiced. The Bible does not present us with a God who changes from Old Testament to New. Just the opposite. It presents us with one unchanging God whose love and wrath are both ratcheted up as they pass from the Old Testament to the New. At the cross, the love of God and the wrath of God shake hands. The cross sums up the Old Testament. It glorifies and expresses both God’s wrath and love.

As I shared this with Tom he gained more confidence in the unity of Bible. He also grew in the fear of God, even as he rested in the love of God in a completely new way. Hopefully you will do the same.