The Roman Empire in the Fourth Century AD saw Christianity established as the state religion. On coins, evidence of this change was rather minor.
A small Chi-Rho on a standard held by soldiers was a common reverse type. The Chi-Rho monogram, or Christogram, combined X and P, the first two letters in the name of Christ.
Only once was there an issue of coins with a prominent use of the Christian symbol. In 353 AD, near the end of his losing battle with Constantius II, the usurper Magnentius
issued bronze coins showing the Chi-Rho between the letters Alpha and Omega (REV. 22:13).
Magnentius / Christogram - AE 2 - 353 AD - 23mm - 5.6g - Arlate mint
Magnentius used the symbol of Christ to solicit support from Christians in the Western Empire in his struggle with the Eastern Emperor Constantius II. Constantius was quite interested in theology and supported the teachings of the Alexandrian heretic Arius
who had been excommunicated at the Council of Nicaea. While popular in the East, the Arian heresy was strongly opposed in the West where the teachings of the Bishop of Rome prevailed.
Arius taught that Christ was not of the "same" substance but was only "like" the Father. Arius emphasized the human aspects of Christ. Use of such a strong Christian symbol would remind people of the danger
a heretic Emperor could be to their beliefs and their souls. Magnentius' interest in this symbol was political and strategic rather than philosophical. Magnentius himself was a pagan.
DNMAGNEN - TIVSPFAVG / SALVS DD NN AVG ET CAES alpha - omega ( ) AR
The reverse legend includes an unusual use of a plural abbreviation. Domini Nostri, our lords, is abbreviated DDNN doubling each letter. These words modify the phrase AVG ET CAES, Augustus and Caesar. Since there was only one Augustus (Magnentius) and one Caesar
(his brother Decentius) credited, the final letters of these words were single (AVG not AVGG). Salus, in this case, wishes for the well being of the pair. Both were dead within weeks of this issue.
As a footnote:
These coins were issued from half a dozen cities in the regions controlled by Magnentius. Arlate began production late in the reign as Constantius was about to end the usurpation. When the forces of Constantius took the mint city of Treveri, a few coins were issued combining
the portrait of Constantius II and the Magnentius type of the Christogram. Remembering that the purpose of this type was to draw attention to Constantius' heresy, this issue was recognized as an error and was soon stopped. In addition to the 'normal' mints, there are many coins of Magnentius that are barbarous style and were probably
produced to fill local needs.