In the Bible, we hear about humanity as we relate to God. In a way, humanity grows up during the Bible. Like little children, God started by giving us absolutes, because humanity could not understand. Just like parents have to give little children absolutes.
As humanity continued to grow up through the years, some absolutes were taken away, since they had served their purpose. The Israelites were ordered not to eat certain meat, yet God removed this through a vision by the first pope, St. Peter. [Acts 10:9-16]
Also, as humanity continued to grow up, some things were given to us. Parents give their children toys and as the children grow up, they give them greater things. The great bread of the Old Testament (the manna from Heaven) was overshadowed by the great bread of the New Testament (the Eucharist).
As humanity grew up, we were given reasons for what God asked of us. As we were able to understand, He gave us understanding. Just as parents do to their children. God parented humanity from infancy to now. He has not gone from vengeful God to merciful God. God does not change, we have.