Why does God seem so different in the Old Testament versus the New Testament?
God saves by grace and by faith, both in the Old Testament and the New Testament.
The Word of God is very clear: God saves by grace and by faith, both in the Old Testament and the New Testament. The Bible says that it was by faith that Abraham was saved (see Rom. 4:3). Abraham lived 400 years before Moses. He didn't have the Ten Commandments; he didn't have all the laws. So, God's way has always been and always will be "by grace . . . through faith" (Eph. 2:8).
Take somebody under the old covenant who has kept all the laws, but his heart is not right with God—that will not save him. Or somebody under the new covenant who goes to church every time the door is open, but he has not surrendered his life to Jesus Christ—that will not save him. Only by faith and grace will God save.
Our God is unchanging. He fully revealed Himself in Jesus. As Jesus said, "Anyone who has seen me has seen the Father" (John 14:9). And that's why Jesus is the full revelation of God. No one before Him and no one after Him can ever claim that.