The Chi-Rho superimposes the first two Greek letters of Christos, X, chi, and P, rho, into a single monogram, one of the oldest Christograms, in Christian use by the early fourth century and possibly before. Its rise is bound to the emperor Constantine: the Christian writers Lactantius and Eusebius report that before the battle of the Milvian Bridge in 312 he was directed by a vision or dream to mark his soldiers with a heavenly sign, and the labarum, the imperial military standard, thereafter carried the Chi-Rho. The accounts differ in detail and cannot be fully reconciled, which is itself worth noting: the tradition that fixed the emblem to Constantine's victory was written down by partisan authors after the fact.
The letters chi and rho had circulated earlier still as an ordinary scribal abbreviation in Ptolemaic and Roman Egyptian documentary papyri, marking words beginning with those letters, but no verified specifically Christian Chi-Rho monument is older than Constantine's reign, so claims of pre-Constantinian Christian use rest on inference rather than surviving artifacts. A related but distinct sign, the staurogram, overlaying tau and rho so the crossbar reads as outstretched arms, is demonstrably older: it appears embedded in the text of New Testament papyri such as Bodmer P66, copied around the year 200, making it Christianity's earliest surviving visual reference to the crucifixion, a full century before the Chi-Rho's public debut.