Jesus Christ is God, who became a man and walked among us and taught us many things. When we consider the kinds of things that Jesus said and did, it's not uncommon for people to view God as having had a “change of heart”. People generally know Christ for His love and compassion as He healed the sick and commanded us to love even our enemies. Conversely, people generally characterize the God of the Old Testament as wrathful, destroying mankind with a flood, slaying the firstborn sons of Egypt, and commanding the death sentence for violations of His Law. Now, the wrathful nature of God may be a concern for some us, personally; but a change in God's character should really be a concern for all of us! If God were to change character from the Old Testament to the New, I think we could rightly say that the New Testament writers were inventing a new God. But what we find, of course, is that God does not change. In every book of the Old Testament He is characterized both by His wrath which is an expression of His justice, and His mercy which is an expression of His love. And in every book of the New Testament, including the gospels describing Jesus' life, we find Christ also to be characterized both by His love and by God's perfect justice.
God's justice and wrath are not in conflict with His love. In the Old Testament we see God's love for Israel in leading them out of Egypt. At the same time we saw a demonstration of God's wrath on Egypt because of their idolatry. Throughout the remainder of the Old Testament there are continual reminders of God's love in this act of delivering Israel from their slavery in Egypt. Typically we are reminded of this whenever a prophet is about to issue a stern warning to the people of the consequences of their sin. It is a reminder of the love of God in order to draw them back to God, for when we truly understand the love God has for us, we naturally respond with love in return, which manifests itself in repentance and obedience. But it is also a reminder of the wrath of God on those who are rebellious against Him. When Israel was brought out of Egypt they were to eventually make war with the Canaanites. Why was this? In Deuteronomy 18:12 we're told it is because of the sins of the Canaanites. But as Israel wandered through the wilderness, we find that time and again when Israel rebelled against Him with idolatry and other offenses, God judged them in much the same way as Egypt and Canaan were judged. During this early period between the exodus from Egypt and the conquest of Canaan, the intensity of God's wrath seems so much greater than in later Biblical history, up until we get to the Assyrian and Babylonian captivities. Why is this? It is because God is establishing the nation; they need to know about the just character of God as well as knowing about His love. Such strictness was not really seen again until the Assyrian and Babylonian captivities, even though the nation fell into far worse sin and idolatry than during those 40 years in the wilderness, even sacrificing their children to idols. So while God's wrath is a dominant theme in the Old Testament, it's actually a testimony to God's love and mercy that we don't see more of it.
Now in the New Testament, we do see God's love manifested to a far greater degree than we had before, because in the New Testament we see God Himself become a man to die for all of us sinners. But even when we think of Jesus' good deeds in feeding the poor and healing the sick, He rebukes the religious leaders of the time because they had failed to practice the Law of the Old Testament; for if they had, they would be doing such good works themselves. The Old Testament is full of the wrath of God because the people of God brought it upon themselves. But this is no different to the New Testament. Jesus says to the religious leaders, the Pharisees, “Thus you witness against yourselves that you are sons of those who murdered the prophets. Fill up, then, the measure of your fathers. You serpents, you brood of vipers, how are you to escape being sentenced to hell?” (Matthew 23:31-33) The wrath of God has not diminished in the New Testament. In fact, we might say it, too, is being revealed to a greater degree than before.
In the Old Testament we saw how Israel, themselves, were judged in the wilderness and incurred God's wrath upon them. And I commented how this revelation of God's wrath was more intense at that time in order to establish the nature of God for the new nation. For us in the New Testament, these lessons still apply. Jude says “Now I want to remind you, although you once fully knew it, that Jesus, who saved a people out of the land of Egypt, afterward destroyed those who did not believe.” (Jude 1:5) Since we have the revelation of the Old Testament, such an intense demonstration of God's justice and wrath is not as necessary. Still, while the Church was in its infancy, we read about how, in Acts 5, Ananias and Sapphira were put to death directly by God for their deception; a sobering reminder of how seriously God takes sin. Or how God put many people to death because, according to 1 Corinthians 11, they were participating in the Lord's Supper “unworthily”. God is no less just and wrathful in the New Testament, but He reveals it to us because He loves us. It's so that we will understand just how loving and merciful He is toward us. I mean, I wonder how often I should have died after taking the Lord's Supper?
Jesus is eternally loving and eternally just, from the beginning to the end...
The one who conquers will have this heritage, and I will be his God and he will be my son. But as for the cowardly, the faithless, the detestable, as for murderers, the sexually immoral, sorcerers, idolaters, and all liars, their portion will be in the lake that burns with fire and sulfur, which is the second death. (Revelation 21:7-8)