Why early church history matters
313 AD may be the year the church stopped following Jesus' example
Relative pluralism, with a regular ritual focus, and an agreed set of texts as a basis for teaching and exploration, was appropriate to a Church which lacked any notion of itself as a single institutional unit, and whose communications were necessarily private and piecemeal. Before Constantine, the Church was simply not in a position to make universally binding and enforceable decisions.
Rowan Williams, Arius: Heresy and Tradition 
It's unclear to me how many Christians realize that what we call Christianity today is based on a series of decisions made by human beings. Many of the features of what “everybody knows” about God and Jesus are not at all obvious from the scriptures and other sources that we have.
Millions of people, especially non-Christians, read The DaVinci Code and so they think they have the scoop on all this. Don't read The DaVinci Code if you want actual historical information; instead, try the book by Rowan Williams . The real story is a lot more complicated than is described by Dan Brown.
In my opinion, the best way to understand the history of how a rag-tag group of followers of Jesus turned into the Universal (Catholic) Church is to think of an analogy to a start-up company that turns into a huge corporation. Let's use Apple as an example. As everyone pretty much knows, Apple was started by a charismatic leader (Steve Jobs) and the guy that did the nuts-and-bolts work (Steve Wozniak). When the company was young, decisions were made mostly based on inspiration and faith; later, as the company matured, decisions became a lot more – well, corporate. This is a well-known phenomenon of growing organizations.
The same thing happened in the early Christian movement (before it became the early Christian Church). Inspirational leaders (first Jesus, then later Paul) attracted people to the message, which is great – but then, other people were left to handle the “nuts and bolts.”
I believe that a good way to think of this is to consider how different people in the church had different responsibilities (although often these responsibilities overlap):
Sacramental: The local priest or presbyter handles the duty of performing the mass. He (sometimes she in the early church) might also have duties to minister to the sick or directly help the poor.
Spiritual: One can say that this is everyone's responsibility in the faith, but fairly quickly this became the purview of ascetics, martyrs and then later monks and nuns.
Doctrinal: This was/is the area for the theologian. In the early church, many theologians were well educated in Greek philosophy as well as scripture.
Organizational: As the church grew, and especially after Constantine made Christianity the state religion, this task fell to the Bishops.
Early on, the role of Bishop was little more than that of a parish priest, but as the church grew, this role became more and more bureaucratic. I think that it helps to realize that not everyone who is good at running an organization is necessarily inspired, spiritual, or steeped in the nuances of philosophy and theology. The thing that bureaucrats desire most is order and normalcy. Well, it's hard to control things when you have people running around being able to say and do anything as the Spirit moves them.
That is the crux of the matter; the nature of organizations is control and regression to the mean. Outliers are problematic. But Jesus himself was an outlier. In my opinion, 313 AD – when Christianity was legalized in the Roman Empire – was the very moment when the church stopped following Jesus's example. This is also an important aspect of what caused the Reformation.
This doesn't mean that there aren't very good things that came from the Church between 300 and 1500 – there were. But some wrong things happened as well. This is why early church history is important.
 Constantine burning Arian books, file: James Steakley; artwork: unknown, Public domain, via Wikimedia Commons
 Arius: Heresy and Tradition by Rowan Williams (links to Amazon)