The God of the Old Testament and the God of the New
Why is the God of the Old Testament so different from the God of the New Testament? The question lies beneath a great deal of the misunderstanding of Theology and Religion the world over. Basically, our Christian roots lie deep within Judaism. The people who were to become the 12 tribes of Israel emerged from a large and fertile triangle of land between the Tigris and Euphrates rivers in what is today Iraq. Over (probably) tens of thousands of years the people in that area spread out and developed into civilisations which were quite complex for their day. We have Syrians, Assyrians, Egyptians, Babylonians, Greeks and a large collection of "wandering Aramians" - Abram (Abraham to be) belonged to one of these.
Eventually, a number of these tribes came together and formed the people known collectively as the Israelites. In time they settled down and began to grow into a civilisation of their own. At that point (about 500 or so BCE) they began to record their experience of God and pieced together a form of their history - a book that we now know as the "Old Testament". The composition lasted right through until the 2nd and 1st centuries BCE. Because of the diverse background of generations they inevitably recounted stories and beliefs from the many (often more advanced) peoples around them. There is a huge collection of Ancient Near-Eastern Texts (known as ANET) which has many of the stories of these peoples recorded. It is remarkable how similar these accounts are. They often speak of the Gods and their interaction with humankind. Almost invariable, humankind comes of worse! Look, for example, at the story of the "Giants" on the website! (Genesis 6:1-4 and 1 Enoch 6-16) There are many wonderful epics of that kind from all parts of the ancient near-east.
The Israelites "borrowed" many of these stories from the people around them. For example the account of the "Flood" which is essentially Babylonian (the Gilgamesh Epic). We see in the OT so many names for "God" which reflect the non-Israelite people from which the stories came, e.g. "El" and "Shadai". The unique characteristic is that, despite the jumble of names and backgrounds and stories, the Israelites INSISTED that their God (Yahweh) was one. "Hear, O Israel, the LORD your God is ONE" - a prayer known as the Shama (it's on the website). What they did not manage to get away from was the fact that their God was still a God to whom allegiance was required (or else!). Another unique feature was the Covenant - an agreement, instigated by God, which identified Israel as the "chosen race" - "You will be my people and I will be your God" (Ezekiel 36:28).
The books of the OT chronicle the unfolding of this covenant and the events which took place when the people wandered away from it! The unshaken faith of the people was, however, that God would not leave them on their own and would eventually "redeem" Israel, vanquish all her enemies and make her the nation "above all nations" (Psalm 113:4 amongst others).
Along comes Jesus. Born into a devout Jewish family in a time of Roman Occupation and after many decades of conflict with more powerful nations around them. One point that Christians tend to forget is that Jesus was a Jew through and through from the top of his head to his toenails! He was taught and fully absorbed the heritage, understanding and expectations of his people. By the time he grew up the tension in Israel had built up such that the people were longing and searching for the chosen "Messiah", the one promised by God who would set the people free. When Jesus began to minister to the people, many saw him as the one!
An aside here: Jesus was not a poor man belonging to the "despised and downtrodden" classes of his people. We are told he was a carpenter - a much valued and sought after trade in a world building cities like there was no tomorrow. He spoke Aramaic, the day-to-day language of the people. He read and spoke Hebrew, the language of the temple and the Scribes and Pharisees. As a tradesman in a world dominated by trade, he would have undoubtedly have understood and probably spoken Greek. When he was hauled before Pilate, he probably conversed in Latin (the nearest Pilate would have come to speaking Aramaic would have been to speak slowly and loudly to these "damn foreigners"). He was able to support himself, with some help from others, as he wandered the country in pursuit of the Kingdom. Jesus was by no means uneducated! Disregard for the moment the concept of the Son of God knowing everything - that is a total red herring and it's not how it works.
So, fully Jewish and steeped in the OT Jesus speaks to the people. They think he might be the Messiah - but his message is very different to the message they were looking for.
Jesus unique understanding of God is that God is "Our Father" (Abba, father) and loves each and every one of us regardless of the mess we might make of things. In time, the disciples he gathered around him began to realise that this new form of Jewishness was very different and eventually they broke away from Jewishness and Christianity emerged. "The grace of Our Lord Jesus Christ, the Love of God and the Fellowship of the Holy Spirit" (2 Corinthians 13:14) became the Christian keynote greeting.
Sadly, however, we still after 2000 years have not quite shrugged off the concept of the "vengeful" God. We are still, in many of our prayers and liturgies, trying to "please" God so that He somehow makes our lives better. Christians, particularly Catholics, have a deep seated feeling of guilt because of our many "sins". Don't get God angry, you never know what might happen!! We go through torments trying to make things better with God.
The point, I think, that Jesus was trying to make was that we should be trying to make things better for each other. Jesus is there with us, alongside us, in trouble with us. God is Our Father (who might sometimes seem far away - that's why Jesus came to be with us always "even to the end of time" (Matthew 28:20)), but he still loves us unconditionally - otherwise would be here at all?