remove the evil of your doings
from before my eyes;
cease to do evil,
learn to do good;
seek justice, rescue the oppressed,
defend the orphan,
plead for the widow.
- Isaiah 1:16-17
When people portray the Old Testament as featuring a vengeful God and the New Testament as featuring a loving God, they are betraying their lack of knowledge of the Bible. It's far more accurate to say that the Old Testament portrayal of God is frequently less affected by Greek thought, whereas the New Testament God has been put through the filter of Hellenism. It is Greek influence that gives us images of God as more distant and less personal. This influence extends back into parts of the Old Testament that were written later, such as the first of the two Creation stories in which God is above all and through all. We can tell that the second Creation story is more ancient because God gets down into the mud to make the first human being. If God is omniscient and omnipresent, then neither image of God is false.
But the point is that the God of Love is there from the beginning. The difference is that in more ancient Hebrew thought, God is more likely to be portrayed as showing that love through righteous anger. We love because we care, and we get angry because we care. When the prophets show God about to smite the people with vengeance, we might imagine a parent getting ready to distribute spankings on disobedient children (which, whatever we may have rightly learned about the effects of corporal punishment on children, is one way that parents have applied discipline for nearly all of human history).
Here, though, we see God's love revealed: This is how I want you to act! Cease to do evil! Learn to do good! This has been my message all along!
Love did not become a new notion to God with the coming of Jesus. Love was the goal all along. But thanks to Jesus, we learned to see love in a new way--and would you look at that? In the New Testament, God is also personal, and not at all distant, and gets angry. All these things Jesus was and is to us.