It may seem that God was more wrathful in the Old Testament than in the New Testament, but that is not the case when we study the whole story of the Bible. In fact, the story of the Old Testament is about God’s continued mercy to Israel despite Israel’s continual rejection of God. Even when God disciplined Israel or sends them into exile (Deut. 28), He graciously called them back and forgave those who repented (Isa. 40:1. 2).
Furthermore, the central place that the cross of Jesus Christ has in the biblical story shows us that God was not actually more wrathful in the Old Testament than in the New. God’s wrath on Israel in the Old Testament is measured and momentary, often mixed with mercy and patience. But in the New Testament, Jesus endured the full wrath of God for sin on the cross for all of those who would be redeemed, including Old Testament saints. In this way, the New Testament displays more of God’s wrath than the Old Testament because God’s full wrath was poured out on Jesus Christ at the cross.
Also, God might sometimes seem more wrathful in the Old Testament because He intended the fullness of His mercy to be shown in Jesus. The Old Testament gradually builds up to this truth that is finally made clear in the New Testament. So, you could say that God’s mercy in the Old Testament was only partially displayed before being revealed most clearly in Jesus Christ in the New Testament. The stress on God’s wrath in the Old Testament prepared God’s people for the full revelation of His mercy in Jesus Christ.
Finally, we cannot forget that the wrath of God is most clearly revealed in the New Testament no less than His mercy is most clearly revealed there. We are told that all those who do not repent of their sins and trust in Jesus alone for salvation will experience not just God’s wrath in history but God’s wrath for all eternity (see Mark 9:42–50; Rom. 5:9; Rev. 20:11–15).