He set a little child in the midst of them
Mathew 18:1 - 3
XP
Constantine the Great, founder of Constantinople, had the monogram of Christ placed on the labarum, or imperial stamdard; It was the Greek letter X (chi) with a P (rbo) placed perpendicularly though it, forming the first two letters of the name Christ, in Greek
IHS
From Pugin's "Glossary of Ecclesiastical Ornament"