Arma Lucis (i.e. "Armor of Light" in Latin) is a reminder of what St. Paul had told us in Romans 13:12: "The night is far spent, the day is at hand: let us therefore cast off the works of darkness, and let us put on the armor of light."
The Arma Lucis logo is inspired by the Latin phrase "In hoc signo vinces" (i.e. "in this sign you will conquer"), which Constantine I proved true as he battled another Roman emperor Maxentius with the first two letters of Christ's name (chi - X, rho - P; Christos) painted on the shields of his soldiers. In a memoir of the Roman emperor that Eusebius wrote after Constantine's death (On the Life of Constantine, circa 337-339), a miraculous appearance was witnessed by Constantine seeing a cross of light imposed over the sun. That very evening, Christ appeared to Constantine in a dream and told him to make a replica of the "heavenly divine symbol" (Latin: coeleste signum dei) on the shields of his soldiers.
Behind the symbolic Chi Rho Roman shield lay a sword in representation of St. Paul who was beheaded like St. John the Baptist. Crisscross with the sword is the key representing St. Peter, whom Christ armored the Church through saying "I also say to you that you are Peter, and upon this rock I will build My church; and the gates of Hades will not prevail against it." (Matthew 16:18).