How did we get to the concept of God that is currently held by evangelical Christianity?
Of course, they would say it came to us from revelation. God revealed himself to man and then inspired man to write down those revelations in documents which were subsequently identified and collated into a single book--the Bible.
Let's take another approach. We know today from neuroscience that human beings are born with a brain that seeks to identify patterns and purpose. This is sometimes called agency detection, (Pascal Boyer) which is the inclination to look for and attribute intentionality or mind or will to happenings (David Eller, Atheism Advanced, p. 93). Studies by Paul Bloom on infant reactions shows that humans, even very young humans, seem to attribute states of mind to things, including other humans and non-human objects. If there is the slightest bit of intelligible behavior, we tend to perceive intentional behavior, an act of mind or will (Eller, p. 92). It is a very small step to decide that the intentional component, which is not the same as the physical part, can exist separately from it and survive its destruction (Eller, p. 92). This is how the concept of the soul or spirit originated.
The oldest and most universal religious belief seems to be animism. Anthropologists have discovered this belief among primitive peoples the world over. Animism is the belief that many physical objects have an invisible and intelligent life force that animates them. These forces are usually called spirits and they cause certain events to take place. Primitives believed, for example, that when a volcano erupted, it was due to the spirit of the volcano that was agitated about something, usually human behavior. People also tended to believe that the life force or spirit of their ancestors survived death and perhaps hung around influencing affairs on earth. Man began to think it was necessary to try to please these spirits in order to facilitate their help or at least prevent their negative actions against them. This led to ancestor worship, offerings and sacrifices to various spirits, and so on.
As the evolution of belief continued, certain spirits or gods came to be associated with virtually every aspect of life. There were fertility gods, warrior gods, health gods,sea gods, weather gods , and so on. When one was going to take a journey on the sea, it was necessary to seek the favor of the sea god. In order to facilitate a good harvest, the fertility god must be placated. This belief in many gods is called polytheism.
As time went on, and stories were told about these gods, they took on many human characteristics (anthropomorphisms). They were seen as interacting with each other and sometimes fighting each other. There developed a hierarchy among these gods. Some were higher than others and one was usually seen as the most high." Another development was for each individual people group to have a tribal god--a god that was in some sense attached to their group. This was the beginning of monolatry or henotheism. This was the idea that only one god should be worshipped even though there were other gods. This idea seem evident in many passages in the OT. It appears that Moses was a henotheist. As the religion of the Hebrews continued to evolve, they adopted monotheism, the notion that there was really only one true God who was the creator and ruler of the whole earth and all other deities were not real. This one, true god continued to have many human characteristics, for example, emotions like love, anger, and jealousy and the ability to repent or change his mind. But this god also had characteristics and powers that were far above man's. He was immortal, very powerful (not necessarily all powerful), perfectly just or righteous and always faithful (Deut. 32:4). The god of the Hebrews also needed to be placated and thus sacrifices were offered to him.
As Christianity came on the scene and adapted many Hellenistic ideas into their concept of this one true god, he took on perfections--omniscience, omnipotence, omnipresence, and omnibenevolence. While some of these characteristics are seen in the Hebrew scriptures, they tend to be expanded human capabilities not idealized perfections as in Greek thought. This brought us to the concept of god that has become standard in evangelical Christianity (as well as most versions of conservative Christianity). This concept is best defined by Richard Swinburne: There exists necessarily a person without a body (i.e. a spirit) who necessarily is eternal, perfectly free, omnipotent, omniscient , perfectly good, and the creator of all things. (The Existence of God, p. 7). In Christian theology, this god had been once and for all placated through the sacrifice of his own son. No longer was it necessary to offer sacrifices to this god. What was required now was for the follower to offer himself as a living sacrifice to the deity (Romans 12:1-2). Thus, the practice of monasticism with its correlate of celibacy originated.
The description of the Christian god continues to undergo modification because its Hellenized character is in many ways contradictory to the god described in the Hebrew-Christian scriptures as well as contradictory to human reason. For example, problems reconciling this concept of god with human free will and the existence of evil in the world has resulted in a new concept of god sometimes called open theism. Of course, evangelicals, for the most part, oppose any modifications to their description of god; but, nevertheless, it illustrates the continuing evolution of the idea of god.
So, I think the evangelical Christian concept of god can be understood as a natural evolution arising from man's need to ascribe pattern and purpose in nature. Beginning with animism, moving through ancestor worship and the attributing of human characteristics to deities all the way to a single perfect deity.