Thank you for the question about the character of God in the Old and New Testament, “Why is God so different in the Old Testament than He is in the New Testament?”
As one reads Exodus through Joshua in comparison to Matthew through Acts, it does appear that the actions of God are different. However as I read the the question, a distinction needs to be made between the character of God and the actions of God.
For one’s consideration, here are my thoughts on this question:
A. God’s actions are different in the Old Testament than the New Testament because Israel lived under a theocracy while Christians live in this world.
God had promised to Abraham that his descendants will live in the Promised Land if they obey his commands. If they obey him, he will protect and provide for them. If they disobey him, he will discipline and even send them into exile. See Deuteronomy 28.
In the formative years of Israel as a nation, God delivered his people from Egypt, provided for them in the desert despite their rebellion against him, and helped them to conquer the Promised Land. God showed his presence and power with them. As one read Judges the curses of God fell on Israel until they cried out to God to deliver them as they repented of their sins. The nation of Israel failed to a priest before the nations. They were exiled from the Promised Land and scattered from 70 CE when Titus destroyed Jerusalem until 1948 when the Jewish State was reestablished, not as a theocracy but as a democracy.
The the formative years of the Christian, Jesus commanded his disciples to make disciples of all nations. They were to be his witnesses from Jerusalem to the different people group of the world (Matt. 28:18–20, Acts 1:8). God showed his power among the followers of Jesus through the indwelling and presence of the Holy Spirit in the believer’s life. They were to proclaim the Good News of Jesus’ resurrection from the grave and his return for his followers. They weren’t to wait for the non-believers to come to them but instead they were to go among the peoples of this world and proclaim Jesus.
Jesus didn’t command his followers to establish a new Promised Land upon the earth. God didn’t give the believers a territory of land whereby Christians are to live on it while they wait for the return of Jesus.
Thus one is living in the Promised Land while the other is living in the world to proclaim Jesus Christ.
B. God’s character is the same in the Old Testament and the New Testament.
When God revealed himself to Moses, he said this about himself in Exodus 34, “So Moses chiseled out two stone tablets like the first ones and went up Mount Sinai early in the morning, as the LORD had commanded him; and he carried the two stone tablets in his hands. 5 Then the LORD came down in the cloud and stood there with him and proclaimed his name, the LORD. 6 And he passed in front of Moses, proclaiming, “The LORD, the LORD, the compassionate and gracious God, slow to anger, abounding in love and faithfulness, 7 maintaining love to thousands, and forgiving wickedness, rebellion and sin. Yet he does not leave the guilty unpunished; he punishes the children and their children for the sin of the parents to the third and fourth generation” (NIV).
The revelations of the Old Testament and the New Testament is for the followers of God. God revealed himself as unchanging in his character whereby the angels declared God is holy throughout the ages. God is faithful to his promises in the Old and New Testament. He will fulfill what he promised to the Jewish people but for now he is gather sheep from among the world’s people to be part of the family of God. God is gracious and merciful to Israel when they departed from his command and even to the local churches who departed from the commands of Jesus. God disciplined and sent Israel into exile. God disciplines and even closed churches’ door when they depart from his word. See Rev. 3–4.
True that God demonstrated his power more outwardly to the Jewish people so that they will know that the Creator God is with them. I believe God’s power is still present in this world. The problem is not God’s power, but believers who quench and grieve the Spirit of God. God has to work more in the believers than through the believers. Our disobedience has led more so to the limited reflection of God’s power. Our lack of faith and submission to God is the problem, not that God is the problem.
God is looking among his people who will fully trust and submit to him. Who knows what the Holy Spirit can and will do that person? Can God use you and me to reflect more of his glory and power through us? By his grace, may we be willing.
SUMMARY: The character of God remains the same, but his power is reflected differently to Israel in the Promised Land than to the followers of Jesus living in this world.
For more perspectives:
https://www.quora.com/Why-is-God-so-different-in-the-Old-Testament-than-He-is-in-the-New-Testament