Before it became the monogram of Christ, the Chi Rho was the monogram of Chronos (whose name also begins with a Chi-Rho), the god of time, and an emblem of several solar deities. In Hebrew, Chi-Rho equates to Tav-Resh. The Chi Rho was used in hermetic alchemical texts to denote time.
This monogram of the Chi Rho is displayed with a wreath symbolizing the victory of the Resurrection, above Roman soldiers, ca. 350.