The Chi-Rho is the monogram of Christ, a combination of the first two characters of the Greek name of Christ, Christós (chi-rho-iota-sigma-tau-omicron-sigma), chi and rho. Tradition records that the Emperor Constantine, who later legalized the practice of Christianity in the Roman Empire, placed the monogram on the shields of his soldiers and his military banners in the Battle of the Milvian Bridge (ad Saxa Rubra, against Maxentius in 313) after seeing it in a dream with the words "en touto Nika," meaning "in this sign you will conquer". The symbol was extremely common in early medieval art, perhaps because it is a non-representational image of Christ and is therefore in accordance with early Christian avoidance of explicit depictions of Christ, which might lead to idolatry.
Exodus 20:4 "Thou shalt not make unto thee any graven image, or any likeness of any thing that is in heaven above or that is in the earth beneath or in the water under the earth: though shalt not bow down thyself to them, nor serve them: for I the LORD thy God, am a jealous God, visiting the iniquity of the fathers upon the children unto the third and fourth generation of them that hate me; and showing mercy unto thousands of them that love me, and keep my commandments."