Why is Xmas an Abbreviation for Christmas?

Why is Xmas an Abbreviation for Christmas?

A friend of the ministry recently found the answer to this question in the prison library.  He mailed this to Simple House because he thought we would share his interest

“Because the Greek letter X is the first letter fo the Greek word of Christ, Xristos.  The word Xmas, meaning “Christ’s Mass,” was commonly used in Europe by the sixteenth century.  It was not an attempt to take Christ out of Christmas.”

His source is The Book of Answers by Barbara Berliner.