X is actually the Greek letter “chi.” Chi is the first letter in the word Χριστός (Christos ), which means “Christ.” X has been an acceptable shortening of the word “Christ” for nearly a thousand years, way before modern day Christians began taking offense at its use. In fact, The Greek letters “X” (Chi) and “p” (Rho) melded together was once a very common symbol signifying Christ and was called the Chi-Rho. The Chi-Rho was also used by scribes to mark a passage, implying that it was “good”.

You can see here Angel Gabriel represented with the letter "X" on his shield also Gabriel is considered as the messenger of God.