"Christian Name"

I was introduced to the notion of a "Christian name" by the movie Jeremiah Johnson. It's one they show repeatedly on TBS. The scene is one in which an Indian named Paints-His-Shirt asks the title character, "What's your name?" "Johnson," he answers. The Indian asks further, "Your Christian name?" "Jeremiah."

Now, I've heard "Christian name" many times since then, but this is the first I've looked up anything about it. The following is from Wikipedia:

The term given name is rarely used in the United Kingdom; forename or Christian name predominate, with the former now used almost universally on official documentation.
The term first name can refer to any forename, not just the very first. In the United States, first name is the most common form, although given name is often encountered on official documents. The term Christian name, on the other hand, has mostly fallen out of favor.
Christian name may refer to the name taken by converts to Christianity upon baptism.

So, it is mostly a British expression that has fallen out of favor. Well, with a name like "Christian" I guess it was bound to wane in popularity!