X is indeed the Greek letter and first letter for Christos.
And so in short Xmas is Christmas.
That being said ‘mas’ on its own has no meaning. Therefore without Christ, christmas means nothing.
Both may very well hold true that Xmas was used in the early centuries as a shorter form for Christmas and also that the secular world simply wanted to remove Christ from the celebration so that perhaps they could make more money by creating their own stories about the festive season or that it would be more inclusive for all those outside of the faith. Then again they had no clue that God our Father loves all His children.
Personally I will always say, wish and spell Christmas in its entire form. This is my own personal reverence and love for my Lord, who through His incarnation, death and Resurrection has given me life to the full. For me the more important thing is that we lead others to this reality. That the Lord our God came to save us and that we should be fully united with and in Him.