Lovers who affectionally sign “XXX”s to valentine cards and letters are usually unaware that the custom goes back to the early Christian era, when a cross mark, or “X,” conveyed the force of a sworn oath. The cross was, of course, a religious symbol. Not only did it refer to the cross of Calvary; it also was the first letter of the Greek word for Christ, Xristos.
In the days when few people could write, their signature cross, or “X,” was a legally valid mark. To emphasise their complete sincerity in an agreement, they often kissed the mark, as a Bible was frequently kissed when an oath was sworn upon it. It was this practice of kissing the “X” that led to its becoming a symbol of a kiss. During World War II, the British and American governments both forbade men in the armed forces from putting “XXX”s on their letters, afraid that spies within the services might begin sending clandestine messages coded as kisses.