Χριστος has been represented by Χρς, or Χς, and by ΧΡ in art and other representation. I have not found the ΧΡ in manuscripts and would not expect it since the manuscript form always includes the grammatical ending.
A quick glance at some facsimiles of Greek manuscripts shows that the words ιησους, χριστος, θεος, ανθρωπος, πατερ, ματερ, πνευμα and some other words were represented by their initial and final one or two letters which represent the grammatical ending. This could be ς,υ,ν,οι, ι &c.
For this reason, I am assuming that the transition from Χς to Χ happened with the beginning of the use of the vernacular languages in Europe, when the ending was no longer relevant. There would be no reason to retain the last letter and X alone came to represent Christ. There is also no reason to see a sign of disrespect in the transition from Χς to Χ. And so Xmas first appeared in English texts in the 16th century.
I’ve always been amused by people who find Xmas a disrespectful abbreviation; all they’re doing is showing their own ignorance of history.