It is like an ancient cartography that is mostly based on guesswork and rumors from those who had traveled some. The maps were beautiful, but not accurate. Most theology is simple guesswork and their "maps" of God and His ways are laughable compared to the reality.
The problem is, how do we know how God can be known? Is there anyone who has actual knowledge of God, without guess work? Forgive me if my solution is prematurely simple (I'm going to speed through my basic assumptions, here), but the only one who has ever really proven his understanding of God is Jesus. He has a set teaching and this teaching is supported by his death and resurrection. Some might dispute the reality of these events, and perhaps I will discuss at another time. For now, I am just going to accept Jesus death and resurrection a priori. This would grant Jesus first-rate status as to understanding God. Ahead of Muhammad, ahead of Buddha (who didn't really claim any special knowledge about God anyway), before the ancient Hindu writings, before Moses or the other prophets. Because none of them has ever been resurrected, thus given the special "God approved" seal.
The next problem is, how do we know Jesus? He doesn't hang around the local church or synagogue or have an internet site. And, since Jesus never wrote anything, we are dependent on the transcription and communication of the early church. The earliest writings are by Paul and he gives some description about Jesus, but little by Jesus. The most helpful early works are the gospels. Not because they are earliest, but because they offer compilations of the earliest apostolic teaching that claim to be the words of Jesus. Although the earliest gospel is complies some 30-40 years after Jesus' death, that writing is quicker than almost all ancient biographies. As far as ancient writings go, they are about as accurate as we can get. And since the words of Jesus are distinctive from other Jewish or Christians teachings of the time, then we can reasonably, if not unquestionably rely on it. Some scholars do, some don't. I don't care what the scholars think anyway-- they've got their own agenda. In my opinion, that's what we got, and it's enough.
Once we accept the gospels as a primary source, then we have two secondary sources to assist us in our understanding of Jesus' teaching and God. First, we have the Hebrew Scriptures (usually called the Old Testament, but that makes it seem so... old, when, from our perspective it isn't really that much older than the New Testament. Ancient is ancient.). The Hebrew Scriptures were used as a source of knowledge about God by Jesus, although heavily re-interpreted by him. Also we have the earliest Christian writings as found in the New Testament. This is the earliest interpretations of Jesus, still in a close social-cultural context to him. Thus, these are helpful in helping us interpret Jesus. However, both of these sources are all secondary and need to be basically understood in light of Jesus' teaching and life. Jesus' clear teaching gives us an outline of God. The rest of Scripture fills in the outline.
So, these are the sources for all accurate theology. There is room for some flexibility, but the foundation is what Jesus says it is. However, that's not the end of the theological process.
Theology is not just a repetition of the most accurate descriptions of God. It is also a cultural explanation and application. Theology is embedded in a particular culture. Thus, Jesus' teaching, in its theological form, should be systematized, described and applied to a particular culture.
The problem of theology, as we found, is culture. If theology begins in culture, it is guesswork. But the process of theology ends in culture, because we all live in culture and we can have no real connection to theology, no real connection to God except through our culture.