There are many people who wrongly contrast the God of the Old Testament with the God of the New Testament saying that the God of the Old testament was an angry God, a God of judgement, and that the God of the New Testament is a loving God, a forgiving God.
This contrast goes back at least to Marcion, a 2nd century false Christian teacher. He said that the God of the Old Testament was a lesser God, even an evil God, and that the Father of Jesus Christ was the true God. Because of this he wanted to take the Old Testament and the Gospels of Mathew, Mark, Luke and John out of the Bible, leaving in an edited version of the Gospel of St. Luke (because Marcion said that the Gospel of Luke was the least “Jewish” of the four) and some of the Epistles of St. Paul.
It is interesting that in Nazi Germany there were people who said that Jesus was not Jewish, and that the whole of the Old Testament should be discarded.
However, in the 2nd century and in the 20th century the Church condemned the idea saying that the God of the both Testaments were the same so that the Old Testament is indeed part of the Bible, although read now in the light of Christ.
A Christian reading of the Old Testament is based on prophecy and typology Many people think that the prophet’s only job was to predict the future. Of course, this is part of prophecy but only one part. We can say that the main role of prophecy was to proclaim God’s word to a concrete people and concrete situations. Sometimes the prophets criticized the people when they worshipped false Gods and oppressed the poor. But when the people of Israel were suffering, perhaps during the Babylonian captivity, God’s prophets proclaimed a message of hope very openly promising a Messiah, a Redeemer who would rescue His people.
Very often in the New Testament writers say that some word of deed of Jesus fulfilled an Old Testament prophecy. This is especially true of St. Matthew’s Gospel. For example:
“Now when they had departed, behold, an angel of the Lord appeared to Joseph in a dream and said, “Rise, take the child and his mother, and flee to Egypt, and remain there till I tell you; for Herod is about to search for the child, to destroy him.” And he rose and took the child and his mother by night and departed to Egypt and remained there until the death of Herod. This was to fulfil what the Lord had spoken by the prophet, “Out of Egypt have I called my son.” (Mt 2:13-15)
Or in the same chapter:
“Then Herod, when he saw that he had been tricked by the wise men, was in a furious rage, and he sent and killed all the male children in Bethlehem and in all that region who were two years old or under, according to the time which he had ascertained from the wise men. Then was fulfilled what was spoken by the prophet Jeremiah: “A voice was heard in Ramah, wailing and loud lamentation, Rachel weeping for her children; she refused to be consoled, because they were no more.” (Mt 2:16-18)
Typology is when some event in the Old Testament is seen to point forward to New Testament events. So, for example, the blood of the lamb which saved the Jewish people from an avenging angel is a type of our salvation through the blood of Christ, or the feeding of the people in the desert with manna is the bread of heaven, a type of Christs last supper and the Divine Liturgy.
Or the Adam of the Old Testament was a type of the New Adam, Jesus Christ (Romans 5:14) “Yet death reigned from Adam to Moses, even over those whose sins were not like the transgression of Adam, who was a type of the one who was to come.”
St. Paul continues this line of thought to 1 Corinthians:
“For as by a man came death, by a man has come also the resurrection of the dead. For as in Adam all die, so also in Christ shall all be made alive. Thus it is written, “The first man Adam became a living being”; the last Adam became a life-giving spirit. But it is not the spiritual which is first but the physical, and then the spiritual. The first man was from the earth, a man of dust; the second man is from heaven. As was the man of dust, so are those who are of the dust; and as is the man of heaven, so are those who are of heaven. Just as we have borne the image of the man of dust, we shall also bear the image of the man of heaven.” (I Cor 15:21-22,45-49)
The first Adam was from the earth, the new, true Adam (Jesus Christ) is from heaven.
So, we can see that the prophesies and typology find their fulfillment in Jesus Christ. Of course the Jewish people do not read the Hebrew Bible in this way, but we Christians have the light of Christ which shows us the deeper meaning of the Old Testament.