When I buy airline tickets, there's usually a box directing me to choose whether I'd like to sit by the window and what type of meal I want (weird because there never is a meal). I can choose the window or aisle seat, or "no preference". I can choose a vegetarian meal, one with low sodium or "no preference". A lot of times, I click "no preference" and sit in my aisle seat with my peanuts and try to endure the weird smells circulating through the cabin.
When you're born, you have some options about what your purpose will or could be. For most people, though, the stewardess has failed to hand you your selection sheet. Without being aware of it, you select "no preference" and you end up serving whatever purpose your parents deem appropriate. When people are born into Christian, Muslim, Hindu or Buddhist homes, they are handed their purpose without ever having viewed their options. In fact, they don't know there are any others... and they assume anyone who doesn't serve the same purpose serves no purpose.
Christianity is like a default purpose; like the meal on the plane that includes meat and a lot of salt. The reason it's the "default" is because it's considered the norm; most people don't have a problem with meat or peanuts and don't need to bother about making sure the crew knows it. Most people (in the US) are Christian and it really occurs to no one that there is any other way to live. They keep railing about how atheists have "no purpose", but the fact is atheists have taken a much more active role in their [flight] experience. Instead of being content with what has been preselected, we are creative enough to come up with our own purposes.
It's obvious, on T|A, that we're all passionate about different things. There are feminists and animal rights activists; there are gay-rights supporters; there are people who simply love travel; there are parents whose purpose is raising their children; there are artists... the list goes on and on and on. We have chosen our purpose instead of letting "the airline" choose for us; instead of letting the clergy or our ancestors choose.
Christianity is the The Universal Purpose (unless, of course, you're in, say, India... or somewhere else); it is a (one) purpose that just happens to be the norm and the default. Christians, when you're ready to be a little more original, there are many purposes to choose from.
Please, don't set all your preferences to "default".