I am surprised at how many Christians do not know much about the history of their own religion, and are angry when atheists and even other Christians use ancient religious symbols for Christ and Christianity. Oftentimes people write "Xian" for "Christian", "Xianity" for "Christianity", and "Xmas" for "Christmas", for example. Those who do not understand this symbolism quickly take offense that "Christ" is being left out somehow, when in fact, these words all contain the same meaning, just a quicker and shorter way of writing it.
Almost the entire Old Testament was written in Hebrew during the thousand years of its composition. But a few chapters in the prophecies of Ezra and Daniel and one verse in Jeremiah were written in a language called Aramaic. The New Testament, however, was written in Greek. This seems strange, since you might think it would be either Hebrew or Aramaic. However, Greek was the language of scholarship during the years of the composition of the New Testament from 50 to 100 AD. No one at that time had a problem with the name of Christ being represented with the Greek symbol "X".
In addition to written language of religious texts, Wikipedia tells us that Christian symbolism is also used to represent central concepts of the Christian faith, either as a reminder of those concepts or as a way of spiritually connecting with the underlying concept or act.
So, for all those who complain about how "sacreligious" those of us who sometimes use the terms "Xianity" or "Xian" in our dialogue, here is an explanation for you:
"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 (chi) 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.