Bishops of Rome under Constantine the Great
Constantine the Great's (272–337) relationship with the four Bishops of Rome during his reign is an important component of the history of the Papacy, and more generally the history of the Catholic Church. The legend surrounding Constantine I's victory in the Battle of the Milvian Bridge (312) relates his vision of the Chi Rho (☧) and the text in hoc signo vinces in the sky and his reproducing this symbol on the shields of his troops.
Source: Wikipedia — Bishops of Rome under Constantine the Great (CC BY-SA 4.0)