This article is from the Atheism FAQ, by mathew firstname.lastname@example.org with numerous contributions by others.
"What does the abbreviation 'xian' mean? Is it an insult?"
When writing the name "Christ", it is quite common to abbreviate it to
X or x, representing the first letter (kai) of the Greek XPICTOC
khristos. For example, "xmas" is a common abbreviation of "Christmas".
"Xian" just means "Christian".
According to the Oxford English Dictionary, the use of the
abbreviation "xian" or "xtian" for "Christian" dates back at least as
far as 1634. Before that, it was more usual to take the first two
letters of XPICTOC, and write "xpian" for "Christian". Priests would
record Christenings using the shorthand "xpen" or "xpn".
So no, it's not an insult.