Well, X sorta looks like The Cross (crucifix), and the Greek letters Chi (pronounced Ki, and is written as X) and Rho (pronounced like rho, and written like P) are the first two letters of Christ in Greek and become the Chi-Rho symbol. So X has symbolized both Christ and the Cross for some time, and then it just becomes cross (like pedestrian crossing). X is also (one of) the most widely used algebraic characters, you use it to signify the unknown in an equation (but also something that does have a specific value), and so it sorta carries over into language (whereas all the other algebra characters sorta stay in math, unless you need characters in addition to x, and then it usually goes x,y,z,n,a,b,c.. but you can do whatever you want, it doesn’t have to be that way).