Why is God so different in the Old Testament than He is in the New Testament?"
 Answer:  At the very heart of this question lies a fundamental misunderstanding  of what both the Old and New Testaments reveal about the nature of God.  Another way of expressing this same basic thought is when people say,  “The God of the Old Testament is a God of wrath while the God of the New  Testament is a God of love.” The fact that the Bible is God’s  progressive revelation of Himself to us through historical events and  through His relationship with people throughout history might contribute  to misconceptions about what God is like in the Old Testament as  compared to the New Testament. However, when one reads both the Old and  the New Testaments, it becomes evident that God is not different from  one testament to another and that God’s wrath and His love are revealed  in both testaments.
 
For example, throughout the Old Testament, God is declared to be a  “compassionate and gracious God, slow to anger, abounding in love and  faithfulness,” (Exodus 34:6; Numbers 14:18; Deuteronomy 4:31; Nehemiah 9:17; Psalm 86:5, 15; 108:4; 145:8; Joel 2:13).  Yet in the New Testament, God’s loving-kindness and mercy are  manifested even more fully through the fact that “God so loved the world  that he gave his one and only Son, that whoever believes in him shall  not perish but have eternal life” (John 3:16).  Throughout the Old Testament, we also see God dealing with Israel the  same way a loving father deals with a child. When they willfully sinned  against Him and began to worship idols, God would chastise them. Yet,  each time He would deliver them once they had repented of their  idolatry. This is much the same way God deals with Christians in the New  Testament. For example, Hebrews 12:6 tells us that “the Lord disciplines the one he loves, and he chastens everyone he accepts as his son.”
In a similar way, throughout the Old Testament we see God’s judgment and  wrath poured out on sin. Likewise, in the New Testament we see that the  wrath of God is still “being revealed from heaven against all the  godlessness and wickedness of men who suppress the truth by their  wickedness” (Romans 1:18).  So, clearly, God is no different in the Old Testament than He is in the  New Testament. God by His very nature is immutable (unchanging). While  we might see one aspect of His nature revealed in certain passages of  Scripture more than other aspects, God Himself does not change.
As we read and study the Bible, it becomes clear that God is the same in  the Old and New Testaments. Even though the Bible is 66 individual  books written on two (or possibly three) continents, in three different  languages, over a period of approximately 1500 years by more than 40  authors, it remains one unified book from beginning to end without  contradiction. In it we see how a loving, merciful, and just God deals  with sinful men in all kinds of situations. Truly, the Bible is God’s  love letter to mankind. God’s love for His creation, especially for  mankind, is evident all through Scripture. Throughout the Bible we see  God lovingly and mercifully calling people into a special relationship  with Himself, not because they deserve it, but because He is a gracious  and merciful God, slow to anger and abundant in loving-kindness and  truth. Yet we also see a holy and righteous God who is the Judge of all  those who disobey His Word and refuse to worship Him, turning instead to  worship gods of their own creation (Romans chapter 1).
Because of God’s righteous and holy character, all sin—past, present,  and future—must be judged. Yet God in His infinite love has provided a  payment for sin and a way of reconciliation so that sinful man can  escape His wrath. We see this wonderful truth in verses like 1 John 4:10:  “This is love: not that we loved God, but that he loved us and sent his  Son as an atoning sacrifice for our sins.” In the Old Testament, God  provided a sacrificial system whereby atonement could be made for sin.  However, this sacrificial system was only temporary and merely looked  forward to the coming of Jesus Christ who would die on the cross to make  a complete substitutionary atonement for sin. The Savior who was  promised in the Old Testament is fully revealed in the New Testament.  Only envisioned in the Old Testament, the ultimate expression of God’s  love, the sending of His Son Jesus Christ, is revealed in all its glory  in the New Testament. Both the Old and the New Testaments were given “to  make us wise unto salvation” (2 Timothy 3:15). When we study the Testaments closely, it is evident that God “does not change like shifting shadows” (James 1:17).