Perhaps it was a small step from believing in a God responsible for one element to believing in a God responsible for all the elements, for creating the world itself. It was another small step from believing in a God who had the best qualities of one species to believing in a God who embodied pure powers without the limitations, from believing in one God among many to believing that only one God exists. However, though the majority of the world's population now believe in one all-powerful God, there are still many different understandings of the nature of that God.
In the Old Testament
The Hebrew Bible, or Old Testament of the Christian Bible, reflects one of the oldest monotheistic religions. Close study of the text illustrates the development of ideas about God over a period of over 500 years. Scholars believe that the concept of God was influenced by the experiences of the Hebrew people and by neighbouring cultures' religions. The Babylonian Exile (sixth to fifth centuries BC) marked the most dramatic change in the concept of God. Whereas pre-exilic texts such as Amos and Hosea reflect an anthropomorphic God, like a super-king, post-exilic texts such as Jonah and Daniel suggest that God is inscrutible, all-powerful, all-knowing and eternal and that all other "gods" simply don't exist.
In Greek philosophy
It was in Greece that the first philosophical concept of a single God was worked out. Epicurus (341-270 BC) argued that a single God would have to be all-powerful (omnipotent), all-knowing (omniscient) and all-good (omnibenevolent) and this definition of God came to be adopted by philosophers of religion from Jewish, Christian and Muslim backgrounds over time. However, although Epicurus' definition makes logical sense, it does not always fit in with the God described in sacred texts such as the Bible and Qur'an.
The problem of evil
What does it mean to say that the creator-God of Judaism, Christianity and Islam is all-powerful when evil so obviously exists in His creation? JL Mackie pointed out that an all-powerful God could have created a better world than this! What does it mean to say that the God of Judaism, Christianity and Islam is all-knowing when those religions teach that human beings are free and therefore morally responsible for their actions? How can we be free when God knows what we are going to do? What does it mean to say that the God of Judaism, Christianity and Islam is all-good when he allows people to suffer so horrendously and makes the business of having and living with faith so very difficult? Surely a good God would not want people to suffer and would make it easy to earn a heavenly reward? These questions have puzzled philosophers of religion and theologians for millennia.
God self-limiting or distant
Some have reconciled the philsophical concept of God with the reality of their faith and experience by seeing God within time and confined by the basic laws of logic and possibility. This God is everlasting. He may be the creator of all, but once he has created things one way he is not able to change the characteristics of His creation. If God creates human beings free he cannot know the future or intervene to prevent the consequences of their actions.
Others are not satisfied with this approach however, and see God as all-powerful, all-knowing and all-good, unaffected by any limitations. The most usual way of retaining God's perfection is to interpret his existence as eternal, outside time and space. Such a God would be wholly simple, perfect and unchanging, unaffected by anything. His power is seen in terms of having created the framework of time, space and possibility itself, His knowledge in terms of creation as a whole rather than resulting on reflecting on aspects within time and space and His goodness in terms of having created a universe that perfectly fulfills God's design in creating it. This God is admittedly distant and it is difficult to reconcile faith in a wholly simple God with the business of institutional religion, with worship, prayer, Bible stories, even with our moral characters and judging us fit for an afterlife.
A solution in the Trinity
Within the Roman Catholic Christian tradition faith in a wholly simple God is made possible by the doctrine of The Trinity. God is of one substance (homoousion) but can be understood through three persons. The Father is eternal but through the Son understands our human condition, what it is to be time-limited and frail, and gives everything to make our situation better out of his great love. The Holy Spirit works in all times and in all places to inspire and strengthen people, to answer prayers, affirm belief, sanctify worship, even to change history.
The nature of the Trinity
It is important to understand that Christians do not believe in three Gods - rather in one perfect God, so far above human comprehension and the normal limits of existence that He has to be understood through the analogy of three persons. No one person of the Trinity is greater than any other, nor is any one person adequate to express the whole nature of God. The mystery of the Trinity is revealed to help the limited human mind go beyond normal concepts and begin to grasp the divine nature.