Dies Natalis Solis Invicti
The Romans held a festival on December 25 of Dies Natalis Solis Invicti, "the birthday of the unconquered sun." December 25 was the date after the winter solstice, with the first detectable lengthening of daylight hours.
The title Sol Invictus had also been applied to a number of other solar deities before and during this period. The type of Sol Invictus, though not the name, appears on imperial coinage from the time of Septimius Severus onwards.
Though many Oriental cults were practiced informally among the Roman legions from the mid-second century, only that of Sol Invictus was officially accepted and prescribed for the army.
Sol Invictus ("unconquered sun") was a religious title applied to at least three distinct divinities during the later Roman Empire: the aniconic Elagabalus local to Emesa, put forward (unsuccessfully) as the head of the official pantheon by his namesake emperor; to Mithras; and to Sol.
There was an earlier, agrarian cult of Sol Indiges ("the native sun" or "the invoked sun" - the etymology and meaning of the word "indiges" is disputed).
Whether the 'Sol Invictus' festival has a "claim on the responsibility" for the date of Christmas (Catholic Encyclopedia (1908)) has been called into question by Joseph Ratzinger, now Pope Benedict XVI, who challenged this theory by arguing that a December 25th date was determined simply by calculating nine months beyond March 25th, regarded as the day of Jesus’ conception (the Feast of the Annunciation). Problematically for Ratzinger, the human gestational period is not nine months, but in a range between 38 and 42 weeks. However, just as Christmas coincides with the winter solstice, the March 25th date neatly coincides with the vernal equinox, and its pagan ritual themes of fertility and sexual congress with nature, that were later associated with Christianity and Jesus. Other recent Christian commentators also agree that the identification of Christ's birthday pre-dates the Sol Invictus festival, noting the earliest record of the celebration of Christ's birthday on December 25 dates to 243 A.D. December 25 is 4 days after the winter solstice (from latin solstitium, "the sun stays still"), and in this period the days start becoming longer and the nights start becoming shorter. So, December 25 could have been chosen as the day of the rebirth of the sun. Some Christians accept the idea that Sol Invictus may be behind the date of Christmas, with the idea that the early church "baptized" the holiday by imbuing it with a new, Christian meaning. In the 5th c., Pope Leo I (the Great) spoke of this in several sermons on the Feast of the Nativity. Here is an excerpt from his 26th sermon:
But this Nativity which is to be adored in heaven and on earth is suggested to us by no day more than this when, with the early light still shedding its rays on nature, there is borne in upon our senses the brightness of this wondrous mystery.
But this sermon was not in any way related to Sol Invictus directly.
In his 22nd sermon, he directly addressed those who attributed the Nativity to Sol Invictus:
Having therefore so confident a hope, dearly beloved, abide firm in the Faith in which you are built: lest that same tempter whose tyranny over you, Christ has already destroyed, win you back again with any of his wiles, and mar even the joys of the present festival by his deceitful art, misleading simpler souls with the pestilential notion of some to whom this our solemn feast day seems to derive its honour, not so much from the nativity of Christ as, according to them, from the rising of the new sun . Such men's hearts are wrapped in total darkness, and have no growing perception of the true Light: for they are still drawn away by the foolish errors of heathendom, and because they cannot lift the eyes of their mind above that which their carnal sight beholds, they pay divine honour to the luminaries that minister to the world.
In this sermon, Pope Leo I clearly establishes that the two feasts were held on the same day, but that they are also not related.
Solar symbolism was popular with early Christian writers. This is also apparent in the prayers and hymns of the Church, such as the Eastern Orthodox Troparion of the Nativity:
Thy birth, O Christ our God, rose upon the world as the light of knowledge; for through it those who worshipped the stars were taught by a star to adore Thee, the Sun of Righteousness, and to know Thee, the Sunrise from on high. O Lord, glory to Thee.
According to the New Catholic Encyclopedia, 1967, article on Constantine the Great:
"Besides, the Sol Invictus had been adopted by the Christians in a Christian sense, as demonstrated in the Christ as Apollo-Helios in a mausoleum (c. 250) discovered beneath St. Peter's in the Vatican."
Indeed "...from the beginning of the 3rd century "Sun of Justice" appears as a title of Christ". Some consider this to be in opposition to Sol Invictus. Some see an allusion to Malachi 4:2.
The date for Christmas may also bear a relation to the sun worship. According to the scholiast on the Syriac bishop Jacob Bar-Salibi, writing in the twelfth century:
"It was a custom of the Pagans to celebrate on the same 25 December the birthday of the Sun, at which they kindled lights in token of festivity. In these solemnities and revelries the Christians also took part. Accordingly when the doctors of the Church perceived that the Christians had a leaning to this festival, they took counsel and resolved that the true Nativity should be solemnised on that day." (cited in "Christianity and Paganism in the Fourth to Eighth Centuries", Ramsay MacMullen. Yale:1997, p155) However, this statement directly conflicts with what we know of the early Christians, namely, that they were ridiculed, tortured and cast apart from operative society precisely because they would not participate in the pagan feasts and celebrations. The early Christians set themselves directly in opposition to the paganism which ruled the day. "Since Christians worshipped an invisible God, pagans often declared them to be atheists." (cited in "The Story of Christianity, volume 1, The Early Church to the Dawn of the Reformation", HarperCollins Publishers, 1984, p36)
This pagan feast is first documented only in the Chronography of 354, which also contains the earliest certain reference to 25 December as the feast of the birth of Christ.