Written by David Caldwell · 8 November 2025

How the Conquering Sun Became the Conquering Galilean

Sol Invictus, Constantine, and the Christian capture of Rome’s sacred time

By the time Christianity became respectable in the Roman world, the empire had already started thinking like a monotheist.

Not in the sharp, creedal way Christians would later define the one God, but in a looser, imperial way: many provinces, many local gods , and over them all one great, radiant, victorious deity who could belong to everyone. In the later 3rd century that deity was made official. Under Aurelian (AD 270–275), Sol Invictus , the Unconquered Sun , was raised to the status of the god of the empire. He had his temple, his priests, his games, his birthday. In other words, the “one god for all peoples” slot was already occupied.

This is the setting in which Constantine appears, the man we are invited to call “the first Christian emperor.” If you look closely at the sources, on Constantine’s coins, the observance of Sunday and Christmas, at Mithras , a pattern emerges. Christianity did not arrive in a religious vacuum. It stepped into an empire that had already centralised itself around a single, solar, universal god. And then it quietly took over his furniture.

What follows traces that shift: how the conquering sun, the bright emblem of imperial unity, became the conquering Galilean.

A World Already Turning Toward One God

It helps to see that late Roman solar religion was not invented out of nowhere. The habit of putting the sun at the top is old.

• In Mesopotamia the sun-god Shamash sees everything and judges everything. Light is authority.
• In Egypt, solar theology could go almost monotheistic , a single visible giver of life.
• In the Iranian world, Mithra begins as a god of covenant and truth but is easily pressed into solar service when his cult travels west.
• And in the Roman army and frontier provinces, those eastern forms prove popular because they are simple and universal: one high god of light who saves, protects, and judges.

By the time Aurelian formalises Sol Invictus in the 270s, the soil is ready. A sun-god who stands above ethnic and civic cults is the perfect god for a porous, multi-ethnic empire. He is “greater than the other gods” without having to abolish them. You can still have your local shrine, but the empire has its own, towering, solar patron.

That is already a trend to monotheism , the empire learning to say, “all gods and all peoples are, in some way, part of the same divine order, but this one god is greater.”

Constantine’s Two Languages

When Constantine wins power, the Christian story says he did so under the sign of Christ. Eusebius tells us he saw a crosslike sign in the sky and heard the promise “by this sign, conquer.” Lactantius says he was told in a dream to mark his soldiers’ shields with a heavenly sign, probably the chi-rho, the Christ monogram. The new banner, the labarum, was topped with that sign.

But even in the 19th century, writers noticed that Constantine didn’t stop talking like a solar emperor. One Irish paper (1888) described a coin from his reign with the Christian monogram on one side and, on the other, the figure of Phoebus and the inscription “Sol Invictus.” The same article reminded readers that Constantine, after his supposed miraculous vision, still “accepted the Pagan office of Chief Pontiff” and still watched omens. The emperor who marched under Christ’s sign did not feel it necessary to cleanly break with the sun.

This is not double-dealing so much as continuity. He was taking a people trained to honour a universal sun and telling them the universal Christ was the fulfilment of that instinct. It was possible, for a time, to speak both languages.

Why Is Sunday the Sabbath?

Christians had long met on the first day of the week to honour the Resurrection. That practice is early and internal. But the move to make that day a public day of rest came from the emperor, and he expressed it in the old solar style.

On 7 March 321 Constantine ordered that judges, city folk, and most workers were to rest on “the venerable day of the Sun.” Agricultural work could continue, but city life was to pause. The decree does not say “because Christ rose on this day,” or “because the church commands it.” It honours the Sun.

A later letter in the Worthing Gazette (1890), arguing with another correspondent about which day was the true Sabbath, quoted this decree and pointed out something people often overlook: Constantine issued it while still happily bearing pagan titles and minting coins to the sun-god. To that writer, it was obvious: the first legal “Christian Sunday” was in fact a state-backed Sun-day, and the church made the best of it.

Seen from that angle, the question “why do we worship on Sunday?” has a layered answer:

1. Christians were already celebrating the first day.
2. The emperor, brought up in a solar-monotheist empire, gave that day legal honour as the day of the Sun.
3. Over time, the Christian meaning absorbed the solar one.

The fit was so natural that no great quarrel followed. A people already used to venerating the sun could accept venerating the risen Christ on the same day.

Why Is Christmas on the 25th of December?

The same logic appears at the year’s great feast.

The New Testament does not give a date for Christ’s birth, nor does early tradition insist on 25 December. What we do see, by the mid-4th century, is the Western church marking the Nativity on the very day the Romans had celebrated Sol Invictus.

The empire was already feasting at midwinter. It was already rejoicing in the rebirth of light, already holding games “dedicated to Sol Invictus,” as the Emperor Julian himself wrote of the Saturnalia season. When the church in Rome attached Christ’s birth to that day, it was not obeying a secret apostolic calendar; it was capturing a day the state and the people already loved.

A London letter from 1890 traced Christmas customs , plum pudding, evergreens, mistletoe , back to pre-Christian winter rites that celebrated returning life. The writer then noted that in the 4th century “the Christianised Teutons” and the Romans together fixed Christ’s birth on the 25th “the birthday of Sol Invictus… derived from the worship of Mithras.” That’s the whole argument in a single Victorian sentence.

So the answer to “why 25 December?” is straightforward: because when the empire had one god, the sun, that was his day , and when the empire had one God, the Father of Jesus Christ, the church kept the day and changed the name.

A Mosaic in Dorset That Tells the Same Story

If this were only about laws and dates it might feel abstract. But there is a piece of Romano-British art that shows the transition in stone.

At Frampton, in Dorset, a Roman villa produced a sumptuous mosaic floor. Contemporary reports in the 19th century noted that the panels are full of standard pagan decoration , sea deities, dolphins, mythic scenes , but in the focal point of the apse there is the chi-rho, the Christian monogram associated with Constantine. The Victorian writer who described it admitted the rest of the floor “indicates rampant paganism,” and suggested the sign was set in “after the conversion of Constantine.”

Later scholars have seen the same thing more clearly: the room is laid out like a celebration of the created, cosmic world, and the Christian sign sits there as capstone. The old imagery was not smashed; it was crowned. The message is simple: all this colour, all these powers, all this sea and myth , it now belongs to Christ.

That is exactly what Constantine did to time itself. The old feasts remained where they were; a Christian emblem was placed on top.

Julian and the Path Not Taken

If Constantine showed how smoothly Christianity could sit on a solar throne, Julian the Apostate showed what the alternative looked like.

Julian was educated in Christianity but fell in love with the old gods, or rather with a reformed, philosophical, solar paganism. He wanted to revive a worship of Helios/Mithras, to make it ethical, to make it attractive, to make it again the religion of the empire. A late 19th-century review of Alice Gardner’s book on Julian described his public religion as “Romano-Hellenism” and his real devotion as “the philosophy of Solar Monotheism.” That is a perfect name for the current against which Christianity ultimately prevailed.

Julian failed. The reviewer signed off with a line worth remembering: “It is the Christ, not the Galilean, that has conquered.” In other words, the universal, cosmic Christ , not a local village preacher , met the needs of an empire that was already used to a single, high god. Julian tried to keep the conquering sun; history kept the conquering Galilean.

A Monotheism that Grew Out of the Empire Itself

When people say “Christianity brought monotheism to the pagan world,” they are telling only half the truth. The pagan world , or at least the late Roman world , had already begun to think imperially about God. Sol Invictus in the 3rd and early 4th century is the best proof of that. Aurelian didn’t install him as one more cult; he installed him as the god of the Roman state. After him, other eastern high gods and even Mithraic language were folded in. Constantine could still wear the old titles and print the old images because the shape of religion had not changed: one head over many members.

What Christianity did was to give that shape firmer content. Instead of a radiant, masculine, imperial sun, it offered a Father and his Christ, with a historical life, a passion, a resurrection, a moral code, and , crucially , a church that could organise belief across provinces far better than a mystery cult could. The form (one god above the many) was Roman and solar; the name (the God of Jesus Christ) was Christian.

This is why Sunday could be both the Sun’s day and the Lord’s day. This is why 25 December could be both the birth of the light and the Nativity. This is why a villa in Dorset could keep its dolphins and still set a chi-rho in pride of place. The empire was not forced to give up its love of light; it was told that the true light had appeared.

So Who Really Conquered?

From Julian’s perspective, the “Galilean” conquered: the religion he disliked, the one that began in a fringe province, defeated his attempt to revive classical religion. From another angle you can say the sun conquered after all , because the Christian God who prevailed in the 4th century was presented, dated, and celebrated in a thoroughly solar, imperial way.

The truth that ties the two together is this:

by the 4th century, Rome wanted one divine centre for its many peoples;

solar religion had already taught it how to do that;

Christianity walked straight into that space and filled it.

The conquering sun, the Sol Invictus who had been made official god of the late empire, did not vanish in a puff of piety. His day, his feast, his eastward tilt, his language of victory , these were all carried over. What changed was the inscription over the door.

The empire awoke one winter morning to find that its light was still there , only now the light had a face, a story, and, as Julian grudgingly admitted, the power to win.

By the 4th century everyone who mattered in the Roman world had learned, twice over, that the empire works best when it has one sacred centre. First it was the emperor himself: the ruler’s genius was honoured across the provinces, and it trumped whatever local gods you had in your own town. When the raw cult of the emperor became harder to sustain , after bad reigns, Christian criticism, and a general cooling toward deifying mortal men , the same need for a unifying sacred figure was met on a higher, safer level by Sol Invictus. The invincible Sun could do what a compromised emperor could no longer do: be a divine figurehead for all Romans. Julian tried to restore that pattern in openly pagan, solar form. The Christians kept the pattern but changed the person. They said, in effect: yes, one empire does need one divine head , but that head is the God of Jesus Christ. So although Julian did not get his wish, the empire did get what he and his pagan predecessors knew it needed: a single, empire-wide, sacral focus that stood above the local cults.