One of the most common complaints against the Bible is a long list of contradictions, pointed out by skeptics. Chief among these complaints would be how different the God of the Old Testament is from the God of the New Testament.
The God of the Old Testament is perceived to be a God of wrath, blood shed, and judgment. In Genesis, he floods the world and kills everyone and everything with the exception of a boat load of people and animals. He commands very bizarre things, like circumcision of male babies, sacrifice of children, and the killing of entire nations (men, women, and children). Foreign nations that willfully surrendered to the armies of Israel were forced into slavery. The God of the Old Testament seems cruel and irrational.
In the New Testament, a man named Jesus is introduced. It appears that he's against everything the Old Testament God teaches. He says to forgive and to not judge people. He says to be merciful, humble, and righteous. He preaches about a God that deeply loves His people so much that He sent Jesus (His Son) to die for the sins of the world. Upon His death, it was preached that God had made peace with the world. These two Gods seem totally different. What's the deal?
The deal is that there really is no difference between the God of the Old Testament and the God of the New Testament. Although it is perceived, by critics, that the God of the Old Testament is cruel, they fail to see that God created man in his own image out of love. Mankind became corrupt through free will, and God began to judge his people. Often times in the Old Testament, God is gracious to his people. He displays this grace through covenants.
From the third chapter of Genesis onward, it is clear that mankind had become corrupt. The corruption of mankind's once innocent nature resulted in murder, war, adultery, lying, stealing, idolatry, and a multitude of other evils that were a direct attack against the righteous God who created all things. In this respect, mankind deserved to be wiped from the face of the planet. However, starting with Noah, a series of promises were made by God to ensure that mankind would be able to have a relationship to the God that created them. The most significant of the promises was sealed through the giving of laws to Moses.
Jesus said that He did not come to abolish the law but to fulfill it. Jesus issued commands that could be found in the Old Testament Law. He also became a sacrifice to save mankind. This was God's plan out of his goodness.
The promises that God made to humanity, to conditionally maintain His relationship, are known as covenants. Although there are more than two covenants in the Bible, the Christian Bible is best understood by two: the covenant with Moses and the covenant through Jesus. Jesus taught all of the same things as Moses, but He revealed that He would fulfill the requirements of the law through His death. This was not something that Jesus (or the Apostle Paul) made up, it was something predicted in the Old Testament.
Jeremiah 31:31-32, 34b (a passage found in the Old Testament) reads,
“when I will make a new covenant
"This cup is the new covenant in my blood, which is poured out for you" (Luke 22:20).
"For the law was given through Moses; grace and truth came through Jesus Christ" (John 1:17). Grace is shown through the forgiveness of sins, and truth is shown through the knowledge of the one true God.