The term Christ is borne down to us by the descendant of Ancient Greek called Koine Greek, which was the de facto common language of the eastern Roman world, and means “anointed one.” The Greek Χριστός is a translation of the Hebrew Masiah, whence comes our word messiah. Nowadays that term is used more or less exclusively (by Christians in the west,) to refer to Jesus. As such it is not properly a name but a title, and thus we sometimes see Jesus referred to as the Christ. Unlike Latin Greek does have a definite article, so there is nothing irregular about this usage at all.
In Old English, or Anglo-Saxon if you like, it was common to put a title after a name rather than before, thus Eadward cyning for “King Edward” and so on. This is where Tolkien gets his odd-to-modern-ears usage Théoden King used by Gandalf in Meduseld (the language of the Rohirrim as reproduced in The Lord of the Rings is Anglo-Saxon to several decimal places.) Thus it is with Christ, and why we sometimes hear and see “Christ Jesus” rather than the reverse, and why this is also valid usage.
Ancient and medieval writers abbreviated. A lot. Which makes sense when you figure that everything had to be copied by hand in the days before Gutenberg. Names, especially names used often would often be abbreviated by the first letter. The name of Christ, thus, was often abbreviated simply by X, the first letter of Χριστός, which is, incidentally, neither the English c or ch sound, but rather a voiceless fricative of the same class as that of the German Bach. It’s a sound that was used in Anglo-Saxon and is still around in some dialects, but which doesn’t show up much in American English anymore, although you might be surprised. So such usages a Xmas and Xtian, while not formally proper, don’t take Christ out of the word at all; he’s still there, obscured in the X by the depressing state of education in America.
And that, my friends, is your holiday message for the year. Everyone have a safe, jovial and pleasant bundle of festivity, whatever your holiday of choice.