The Sign of the Chi Rho

The sign of Christ is formed from the first two superimposed letters of his Greek title "Christos" (CRISTOS), the first being the Greek letter Chi (X), and the second being the Greek letter Rho (R).

A Roman coin
Obverse: The Roman emperor Constantine
Reverse: The Chi-Rho sign between the letters Alpha (A) and Omega (W)

An essay on this topic is under construction