These phrases don't negate the one God nor that Jesus is God. What they do is speak of our perception of God as depicted in the Old and New Testaments.
God, in the Old Testament, seems to be represented as "making war" with other nations in favour of the Israelites and at other times, enabling other nations to destroy them. This God "requires" killing and destruction, it appears. As a result, we have tended to think of the God portrayed in the Old Testament as being cruel and unjust.
This is in contrast to what we know of the God in the New Testament. Jesus reveals a God who is loving and cares for the little ones and the lost.
But remember the Old Testament is really a love story between God and the Israelites. God made a covenant with them to protect and care for them and they are to obey and be the people of God.
Therefore, these stories really show the strong belief of the Israelites in God for they think that whatever suffering comes to them is the direct result of their sinfulness, their abandonment of God. God sends them these trials to bring them back to their senses.
Although the prophets sometimes speak words of war and destruction, they also speak God's words of forgiveness and consolation. Therefore, in addition to some cruel images, Old Testament texts give us some of the most touching and loving images of God which we frequently use for our prayer.
When the Israelites are suffering or being reproached for their infidelity, God is revealed as a compassionate God who loves them most tenderly. Read through almost any book of the Old Testament and you will find references to God's loving care for the poor, the orphan, the widow, basically anyone in need.
What could be more touching than the loving and gentle parental image in Hosea: "When Israel was a child, I loved him. It was I who taught Ephraim to walk, who took them in my arms; like one who raises an infant to his cheeks, I stooped to feed my child" (11:1-4).
A God who is close to us and initiates a loving contact to which we are invited to respond is seen in Isaiah: "The Lord called me from birth, from my mother's womb he gave me my name" (49:2) and "Come to the water. Come without paying and without cost" (55:1).
When we refer to the God of Jesus, we are saying that this is what God is like as revealed to us in the New Testament by Jesus in his life and teachings. It is true that the Gospels do not always clearly state that Jesus is God but that does not mean that Jesus is not God.
At the Last Supper discourse in John's Gospel, Philip asks Jesus to show them the Father. Jesus responds: "Have I been with you all this time and you still do not know me? How can you say 'Show us the Father'? Whoever has seen me, has seen the Father" (14:9). What stronger words could we have in the Gospels as to Jesus' identity!
Therefore, clearly, the God of Jesus and the God of the prophets is the same God, sometimes seen by humans from a different perspective.
It is the same loving and merciful God who never gives up on humanity.
It is the same God, giver of all good things from the creation story in the first verses of Genesis to Revelation, the last book of the Bible, the God who does not force us but who lovingly and patiently awaits our response "Listen! I am standing at the door, knocking. If you hear my voice and open the door, I will come in and eat with you and you with me" (3:20).
I've seen the phrase God of Jesus and God of the prophets used almost as if they are two different gods. Isn't there only one God and isn't Jesus God? So what do these terms mean?