Chi Rho
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The Chi Rho is one of the earliest forms of the Christogram, formed by superimposing the first two letters—chi and rho —of the Greek ΧΡΙΣΤΟΣ in such a way that the vertical stroke of the rho intersect
Missorium depicting Emperor Constantine the Great's son Constantius II accompanied by a guardsman with the Chi Rho depicted on his shield (at left, behind the horse)
Coin minted during reign of Ptolemy III Euergetes in Egypt (from 246 BC to 222 BC). Chi Rho visible between bird's legs.
This coin of Crispus, son of Constantine, with a chi rho on the shield (struck c. 326) shows that the symbol mentioned by Lactantius and Eusebius was a chi rho.
The so-called "Chrismon of Saint Ambrose" (Chrismon Sancti Ambrosii), on display on the eastern wall of Milan Cathedral, a Chi-Rho combined with Alpha and Omega in a circle. According to Landulf of Milan (12th century), it was used by Saint Ambrose to introduce the catechumens to the mysteries of the Christian faith (whence it was called "oracle" or chresmos of St. Ambrose, written by Landulf as crismon, whence the later Neo-Latin term for the Chi-Rho symbol).
Lactantius
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Lucius Caecilius Firmianus Lactantius was an early Christian author who became an advisor to Roman emperor Constantine I, guiding his Christian religious policy in its initial stages of emergence, and
Fourth-century mural possibly depicting Lactantius (or possibly Apuleius)
Beginning of Lactantius' Divinae institutiones in a Renaissance manuscript written in Florence ca. 1420–1430 by Guglielmino Tanaglia
Page from the Opera, a manuscript from 1465, featuring various colours of pen-work