In Rome December 25 was made popular by Pope Liberius in 354 and became the rule in the West in 435 when the first "Christ mass" was officiated by Pope Sixtus III. This coincided with the date of a celebration by the Romans to their primary god, the Sun, and to Mithras, a popular Persian sun god supposedly born on the same day. The Roman Catholic writer Mario Righetti candidly admits that, "to facilitate the acceptance of the faith by the pagan masses, the Church of Rome found it convenient to institute the 25th of December as the feast of the birth of Christ to divert them from the pagan feast, celebrated on the same day in honor of the 'Invincible Sun' Mithras, the conqueror of darkness" (Manual of Liturgical History, 1955, Vol. 2, p. 67).