The worship of the Sun (Sol) was indigenous to the Romans, who had a temple to Sol Indiges on the Quirinal that was said to have been established by Tatius, king of the Sabines, the first inhabitants of the hill who, after the rape of the Sabine women, reconciled with Romulus and ruled jointly in the eighth century BC.

There also was a temple to Sol in the Circus Maximus, where chariot races took place under the auspices of these deities. The four-horse quadriga, for example, was consecrated to the Sun, just as the two-horse biga was entrusted to the Moon. The foundation dates, too, of the temples on the Quirinal and in the Circus both were in August, when the heat of the sun was most intense.

After the great fire of AD 64, in which a large portion of Rome was destroyed, Nero erected a colossal statue of himself one hundred and twenty feet high, which Vespasian converted to one of Sol, placing on its head a radiant crown. Vespasian also was the first emperor to display the image of Sol on imperial coinage. By the second century AD, this autochthonous deity was being eclipsed by an Eastern cult of the Sun, Invictus appearing as an epithet in an inscription in AD 158. Several decades later, Commodus became the first Roman emperor to appropriate the title for himself.

Septimius Severus, who had command of the fourth legion in Syria, married Julia Domna, younger daughter of the high priest of Sol Invictus Elagabal, whose son Caracalla also adopted the titleInvictus. In AD 219, not long after Elagabalus arrived from Syria, where he had been the hereditary priest of the sun god Elagabal in Emesa, Sol Invictus (the Invincible or Unconquerable Sun) was introduced to Rome as its principal deity. Elagabalus enlarged the Temple of Jupiter Victor on the Palatine and rededicated it in AD 221 as the Elagabalium, where the rites of Jews and Christians were to be transferred "in order that the priesthood of Elagabalus might include the mysteries of every form of worship". Indeed, the emperor, sought "to abolish not only the religious ceremonies of the Romans but also those of the whole world, his one wish being that the god Elagabalus should be worshipped everywhere".

"In every respect an empty-headed young idiot", Elagabalus, impiously having presumed to elevate a foreign god above Jupiter himself, was killed by the praetorian guard and the cult of Sol Invictus suppressed. It was re-established half a century later in less contentious form by Aurelian, whose troops had been inspired by a "divine form" in the Battle of Emesa against Zenobia in AD 272. Victorious, he entered the city and went to the Temple of Elagabalus, where the apparition again appeared to him. Aurelian triumphantly returned to Rome two years later, after recovering the Gallic Empire, and was hailed as Restitutor Orbis, "Restorer of the World." A magnificent temple to Sol was erected, to which great quantities of gold and jewels were dedicated, and a new college of pontiffs established to serve the god, who was to be the supreme deity of Rome.

Games also were instituted, which are recorded in the Chronography of AD 354, an illustrated codex (the first in Western art) compiled that year in Rome as a gift to a Christian aristocrat. In the section known as the Calendar of Philocalus (after the calligrapher whose name appears on the dedication page). (December 25) is annotated N INVICTI CM XXX. Although the dedication is uncertain, the presumption is that Natalis Invicti refers to the birthday of the Invincible Sun (who is mentioned in the games of August 28) and the foundation date of the temple. Thirty races were run in the circus that day. And every four years, thirty-six races were dedicated to Sol on the last day of games that extended from October 19-22, possibly to commemorate the earlier triumphal procession of Aurelian. These quadrennial games are mentioned by Julian (ruled AD 361-363) in his Hymn to King Helios (CLV), who refers to them as a "more recent" institution.

In another section of the Chronography commemorating the laying to rest of martyrs , the liturgical year begins on December 25, and VIII Kal. Jan. is annotated natus Christus in Betleem Iudeae ("Christ was born in Bethlehem of Judea"). In a section listing the consuls, there also is a note for AD 1: dominus Iesus Christus natus est VIII kal. Ian. These are the first references to December 25 as the birthday of Jesus. Since no martyrs are mentioned after AD 336, the first celebration of Christmas observed by the Roman church in the West is presumed to date to that year.

In the Julian calendar, December 25 also marked the winter solstice, the shortest day of the year, after which days begin to lengthen. For Ovid, it was the first of the new sun and the last of the old. Varro, too, regarded the year as beginning on that date. "The time from the bruma [so called because it is the shortest day of the year] until the sun returns to the bruma, is called annus 'year'" .

In 45 BC, it was decreed that public sacrifice should be made to celebrate the birthday of Julius Caesar, a sacrifice that became obligatory after he later was declared a god. The birthday of Augustus, too, was recognized in thanksgiving after his victory at Actium, as were the birthdays of other Julia-Claudians. The dies natales of family and friends were celebrated, as well, with gifts and banquets. Horace remembered the birthday of his friend and patron Maecenas, as was the birthday of Messalla by Tibullus, Macrinus by Persius, Cynthia by her lover Propertius, and Virgil by a reverential Silius .

Unlike the Romans, however, Jews and Christians tended not to recognize birthdays. Late in the first century AD, Josephus remarks that "the law does not permit us to make festivals at the births of our children, and thereby afford occasion of drinking to excess" . Indeed, only two birthdays are mentioned in the New Testament: that of Pharaoh and Herod Antipas, tetrarch of Galilee, whose marriage to his brother's wife Herodias had been denounced by John. When her daughter Salome danced before the king at his birthday feast, she was promised whatever she might ask. At the instigation of her mother, she demanded the head of John the Baptist.

That the gods themselves had birthdays was thought ridiculous as well. Writing about AD 296, Arnobius mocks pagans for anthropomorphizing them. "We men gather our vintages, and they think and believe that the gods gather and bring in their grapes; we have birthdays, and they affirm that the powers of heaven have birthdays".

Mark and Paul make no reference to when Jesus was born, and Matthew and Luke, although they include an account of Jesus' birth, do not mention the time of year. Nor were the early Christian fathers interested in establishing a calendar date.

The Nativity is not mentioned among the "certain days" that should be observed. Nor is it included in the feasts recognized by Tertullian, who, writing in the last years of the second century AD, admonished Christians not to partake in the Saturnalia, or gift-giving at the New Year or midwinter, or "an idol's birthday" when "every pomp of the devil is frequented". "The Saturnalia and New-year's and Midwinter's festivals and Matronalia are frequented presents come and go new-year's gifts join their noise banquets join their din!" (XIV). Just as the heathen does not celebrate the Lord's Day (Sunday) or Pentecost, so Christians should not partake in their festivals; rather, they have a festive day every week whereas pagans celebrate only once a year.

If the birth of Jesus was not celebrated by the early church, it also was because there was not a consensus as to when it had occurred. Writing shortly after the assassination of Commodus on December 31, AD 192, Clement of Alexandria provides the earliest documented dates for the Nativity. One hundred ninety-four years, one month, and thirteen days, he says, had elapsed since then, which corresponds to a birth date of November 18 or, if the forty-nine intercalary days missing from the Alexandrian calendar are added, January 6. Moreover, "There are those who have determined not only the year of our Lord's birth, but also the day", including dates in April and May, as well as another day in January.

Hippolytus, a younger contemporary of Clement, does state that the Nativity had occurred on December 25. Although the statement may be a later interpolation, he reiterated several decades later (in AD 235) that Jesus was born nine months after the anniversary of the creation of the world, which Hippolytus believed to have been on March 25 . The Nativity then would be on December 25.

In about AD 221, Julius Africanus wrote the Chronographiae, the first Christian chronology. Although he does not specifically mention the Nativity, he did believe that Jesus had been conceived on March 25. In AD 243, Cyprian is the first Christian writer to associate the birth of Jesus with the Sun: "O! The splendid and divine Providence of the Lord, that on that day, even at the very day, on which the Sun was made, Christ should be born". Creation itself was on March 25, the vernal equinox, and the Sun created on the fourth day, March 28. It followed, then, that the "Sun of righteousness," in Malachi's phrase, would be born on the same day.

On December 25, AD 380, Gregory of Nazianzus delivered a sermon in Constantinople in which he referred to the day as "the feast of God's Appearing, or of the Nativity: both names are used, both titles given to the one reality....The name of the feast, then, is 'Theophany' because he has appeared, but 'Nativity' because he has been born" (Oration XXXVIII.3).

The Nativity first was celebrated in Alexandria on December 25, AD 432, when Paul, Bishop of Emesa, preached before Cyril on Mary as Mother of God (Theotokos). Eventually, the time between the Nativity and Epiphany became known as the twelve days of Christmas. John Cassian (d. AD 435), writes that the church in Egypt continued to celebrate both the Baptism and Nativity "not separately as in the Western provinces but on the single festival of this day".

Half a century after the Philocalian Calendar had commemorated the first celebration of Christmas in the West, John Chrysostom delivered his homily on the feast day of Philogonius, bishop of Antioch, who had died some sixty years earlier. It was delivered on December 20, probably in AD 386. The day and month are confirmed by the fact that John is anticipating the Feast of the Nativity, which was to occur in five days' time (December 25). That was the day he delivered another homily, In Diem Natalem ("On the Birthday"), in which he remarks that it has been less than ten years since the festival had been introduced at Antioch.

And yet Epiphanius, Bishop of Salamis who died in AD 403, continued to argue that January 6 was the date of Jesus' birth. "Greeks, I mean idolaters, celebrate this day on the eighth before the Kalends of January [December 25], which Romans call Saturnalia....For this division between the signs of the zodiac, which is a solstice, comes on the eighth before the Kalends of January, and the day begins to lengthen because the light is receiving its increase. And it completes a period of thirteen days until the eighth before the Ides of January [January 6], the day of Christ's birth".

The winter solstice, which coincided with the Christian festival, still was recognized however. Leo I (AD 440-461) repeatedly was obliged to admonish the faithful not to honor the sun on the very doorsteps of the old basilica of St. Peter's in Rome, which was oriented so that the sun, rising from the east, would shine in through the doors and illuminate the apse at the west. (If the worshippers had their backs to the Sun, Leo himself still was obliged to face east.)

Even if Christians were worshipping the creator of the sun and not the sun itself, there was concern that pagans would be mislead by the practice and confuse one religion with another. Leo certainly was aware, even in the fifth century AD, of the coincidence between the feast of the Nativity and the winter solstice, and that the honor that should be inherent in the former might be thought to have derived from the latter.

Three centuries later, the papacy still was confronted with the vestiges of pagan custom. In AD 742, Boniface, apostle and archbishop of Germany, reproached Zacharias, complaining that his attempt to convert the heathen there was being thwarted by the behavior of Christians in Rome.

In seeking to determine the date of Christmas, critics have tended to discuss the matter in one of two ways. Adherents of Calculation Theory seek to demonstrate that the Nativity of Jesus can be determined independently by the chronology of the liturgical calendar. Proponents of the History of Religions, on the other hand, tend to interpret Christmas as a substitution for the annual birth of Sol Invictus on December 25. These two approaches need not be juxtaposed, however, nor be mutually exclusive alternatives.

In the Julian reform of the Roman calendar, December 25, the eighth day after the Kalends of January, was recognized as the winter solstice. Nine months earlier, March 25 was the vernal equinox, the eighth day before the Kalends of April, which marked the beginning of spring. This tradition of assigning the equinoxes and solstices to the eighth day before the Kalends (the first day of the month) later was embraced by the church in its calculation of the birth date of Jesus.

Rabbinic scholars had understood the births and deaths of the Old Testament patriarchs to have occurred on the same day. Because Jesus was deemed to be perfect, his life was thought to be complete as well and to comprise a whole number of years. March 25 (the eighth Kalends of April) was believed to be the date of his conception and, exactly nine months later, December 25 (the eighth Kalends of January) his Nativity. The date of Jesus' conception and crucifixion, therefore, were thought to have occurred on the same day of the year: March 25 (the eighth Kalends of April). Fittingly, this also was the day on which the world itself was believed to have been created.

In correlating the conception of John the Baptist with the birth of Jesus, the author of a fourth-century AD tract erroneously attributed to John Chrysostom calculated that Elizabeth (the mother of John) must have conceived on the Day of Atonement, September 24 on the mistaken assumption that her husband Zechariah then served as high priest in the Temple. John's birth, therefore, was presumed to be June 24 (the eighth Kalends of July) and that of Jesus six months later on December 25. The one-day discrepancy between the two dates can be attributed to how the Roman calculated the days of the month. There is one less day in June than in December, as there is when counting the six months between June 24 and December 25.

John was understood to be preparing the way for Jesus, just as the sun begins to diminish at the summer solstice and eventually increases after the winter solstice. Here, then, the Christian feasts were aligned with the four traditional turning points of the solar year: the birth of Jesus on the winter solstice (December 25), the conception (and death) of Jesus on the vernal equinox (March 25), the birth of John the Baptist on the summer solstice (June 24), and the conception of John on the autumnal equinox (September 24). The same liturgical calculation was used by the eastern church, which also believed that Jesus was conceived and died on the same day, April 6 and therefore must have been born exactly nine months later on January 6.

Hijmans presents a critical re-evaluation of the History of Religions hypothesis and the notion that the early church incorporated the feast of Sol Invictus into its own liturgy, positing instead that the pagan festival was "'rediscovered' by pagan authorities in response to the appropriation of the winter solstice by Christianity." The festival of Sol Invictus, in other words, may not have been identified with December 25 until after the first Christmas had been celebrated on that day. Nor, he argues, should the cosmic symbolism attached to the winter solstice, which may have led the church to adopt December 25 for its feast of the Nativity, be confused with a cult of Sol on that date.

The winter solstice, when the light of day finally begins to lengthen, would have a natural association with the "Sun of righteousness." Indeed, Tertullian writes that "It is therefore due to a want of heed and reflection that many are offended by the mere fact that heresies have so much power. How much would they have if they did not exist?". Here, the paradox is that the absence of heresy would confound the predictions of Scripture, as when one is admonished to "beware of false prophets".

Like the cult of Sol Invictus, Mithraism was introduced from the East and perceived by the church to be sufficiently similar to Christianity that it was considered a threat. In the second century AD, for example, Justin Martyr wrote of the Eucharist, "Which the wicked devils have imitated in the mysteries of Mithras, commanding the same thing to be done. For, that bread and a cup of water are placed with certain incantations in the mystic rites of one who is being initiated, you either know or can learn". The mysteries of Mithra, he insisted, were distortions of the prophecies of Daniel and Isaiah, contrived that "the words of righteousness be quoted also by them."

Tertullian, too, was obliged to acknowledge similarities between Mithraism and Christianity but, rather than admitting that Christianity might have adopted certain rites, disparages them as diabolical counterfeits. "Let us take note of the devices of the devil, who is wont to ape some of God things with no other design than, by the faithfulness of his servants, to put us to shame, and to condemn us." .

He also was an apologist for charges that Christians worshipped the Sun.

Rather than juxtapose the notions of Histories of Religions and Calculation Theory as mutually exclusive alternatives or argue whether Christians or pagans were the first to appropriate December 25 as the natal day of their respective god, the winter solstice, when the light of day first becomes ascendant, would seem the natural birthday of both the Invincible Sun and the "Sun of righteousness."