Photo by Formulax
A: Because that's what Christianity originally was -- a movement within Judaism that insisted Jesus of Nazareth was the long awaited Messiah, or "Christ" in Greek. It spread by word of mouth (Acts of the Apostles 8) and publicity (Acts 17.16-34), as movements tend to do, and eventually picked up the possibly derisive nick name "Christ-followers" (i.e., Christians).
The word we translate as "church" due to long tradition started off meaning a civic meeting:
"Is there something else you want to talk about? Then come to the regular town meeting of the people. It can be decided there," (Acts 19.39 ERV).
The word rendered "regular town meeting" is ekklesia, the same word usually translated "church." For Jesus's early followers ekklesia didn't at all conjure up what we think of when we say "church" here on the other end of 2000 years of history. For them "church" was just a gathering, often a secret and rather dangerous one, where they could worship Jesus and learn more about his message from his students.
The purpose of Authentic Light is to teach simple, radical, historic Christianity to interested parties, particularly people who don't follow Jesus yet but might like to. One of the things that gets in the way of that, I believe, is stodgy, traditional, in-house jargon that makes the most interesting thing in the world boring.
To try and crack through that wall of jargon I use different words and phrases like 'Christian Movement' or 'Jesus' Movement' instead of the traditional church lingo.