Merry Christmas!! (A Contrary view)

[Primitivus]

But surely Christian should be christian, until put together with Roman Catholic Christian, or Protestant Christian, or Orthodox Christian, which are to many as different as chalk and cheese.

Not really. The religion or faith is Christian. The adjective identifies denominational association, liturgical tradition, theological preference, etc. When people ask my "religion" (which is even listed on my dog tags), I always respond "Christian." Then I say, "what you probably really want to know is my denominational preference, right?"

Now, as to the diversity of Christian denominations, you're right. There are many. A person who thinks all Christians are alike makes the same error as does an individual who believes all Pagans worship the same deities.