That's not been my experience, so I'm offering some alternative poster ideas:
Peter Rollins offers an interesting alternative to Alpha itself - the Omega course. Here's how Wikipedia describes it:
The Omega Course is a contemplative practice that plays off the large-scale evangelical project called the Alpha course. The Omega Course explores many of the same themes taken up by the Alpha course and does so in a similar manner (via video clips and discussion). However, unlike the Alpha course, the conversation does not direct participants to a conclusion in which people are encouraged to embrace the “right” doctrinal answer—instead, the conversation itself is what is deemed important. Disagreements are encouraged, and the passionate exchange of ideas is affirmed. By the end of the course, people thus experience firsthand that Christianity is not a rigid, monolithic, unchanging system of creeds, but rather a fluid tradition that welcomes interrogation and rigorous discussion.
That sounds like the right kind of course for me. So do the meetings described in the article Has Jesus left Christianity? which talks about a group of people who do not have the label of Christian (in fact, some are agnostics and atheists) who are living out the principles of Jesus and meeting to discuss them!