It is meant to display the event, before the Battle of the Milvian Bridge when at Saxa Rubra north of Rome along the Tiber, Constantine sees a vision of the cross with the words In Hoc Signo Vinces (In this sign, you will conquer).

The phrase appears prominently placed as a motto on a ribbon unfurled with a passion cross to its left, beneath a window over the Scala Regia, adjacent to the statue of Constantine.

[2] Emperors and other monarchs, having paid respects to the Pope, descended the Scala Regia, and would observe the light shining down through the window, with the motto, reminiscent of Constantine's vision, and be reminded to follow the Cross.

The moral of this story would not have been lost upon royal visitors to the pope, or for that matter, Cardinals accompanying a deceased pontiff's cortege, who are meant to see the leader of the church as the embodiment of the divine power that over-rules the kings of the world.

This theme is often repeated in Vatican artworks such as Giulio Romano's fresco of The Battle of Milvian Bridge, located in the Sala di Costantino ("Hall of Constantine") as well as the marble relief in St. Peter's of Algardi's Fuga d’Attila.