The Chi-Rho symbol was used by the Roman Emperor Constantine I (reigned AD306-337) as part of a military standard (vexillum). By the year 350, the Chi-Rho began to be used on Catholic sarcophagi and frescoes.
The usurper Magnentius appears to have been the first to use the Chi-Rho monogram flanked by Alpha and Omega, on the reverse of some coins minted in 353.
A tessellated mosaic paver with the Chi-Rho monogram dating from the 300s was uncovered at Hinton St. Mary, a village in Dorset, England.
Hinton St Mary Mosaic, central panel of a Roman mosaic found at Hinton St. Mary, a village in Dorset, England, on Sept. 12, 1963, by the local blacksmith, Walter John White. It is a large, almost complete Roman mosaic, presently in the British Museum. “On stylistic grounds it has been dated to the 4th century and is attributed to the mosaic workshop of Durnovaria (modern Dorchester).” Photographed by Udimu. https://commons.wikimedia.org/wiki/File:Hinton_St_Mary.jpg