Just as regularly as the approach of Christmas means an increase in the number of advertisements for perfume on television, so also it means that anti-Christian sceptics will proclaim that Christmas was originally a pagan mid-winter festival which Christians appropriated for their own nefarious purposes when they became the dominant religious force in the Roman empire from the fourth century AD onwards.

There are two problems with this claim.

The first is purely logical. Whatever religious festivals non-Christians may or may not have celebrated in the Roman Empire, one thing we can be absolutely certain about is that they did not celebrate Christmas. The clue is in the name. The word Christmas means the liturgical celebration ('mass') held to mark the birth of Christ. Non-Christians did not have such celebration for the simple reason that they were non-Christians.

The second is that there is no early evidence that the early Christians chose to celebrate the birth of Christ on what we call 25 December in order to displace a pagan midwinter festival.

As Andrew McGowan puts it in his recent article 'How December 25 became Christmas': "Despite its popularity today, this theory of Christmas's origins has its problems. It is not found in any ancient Christian writings, for one thing. Christian authors of the time do note a connection between the solstice and Jesus' birth: The church father Ambrose (c. 339–397), for example, described Christ as the true sun, who outshone the fallen gods of the old order. But early Christian writers never hint at any recent calendrical engineering; they clearly don't think the date was chosen by the church. Rather they see the coincidence as a providential sign, as natural proof that God had selected Jesus over the false pagan gods."

He goes on, "It's not until the 12th century that we find the first suggestion that Jesus' birth celebration was deliberately set at the time of pagan feasts. A marginal note on a manuscript of the writings of the Syriac biblical commentator Dionysius bar-Salibi states that in ancient times the Christmas holiday was actually shifted from January 6 to December 25 so that it fell on the same date as the pagan Sol Invictus holiday. In the 18th and 19th centuries, Bible scholars spurred on by the new study of comparative religions latched on to this idea. They claimed that because the early Christians didn't know when Jesus was born, they simply assimilated the pagan solstice festival for their own purposes, claiming it as the time of the Messiah's birth and celebrating it accordingly."

In fact, the most likely explanation for the Christian celebration of the birth of Christ on 25 December from the second century onwards is that this was the date when Jesus was actually born. As the Roman Catholic writer William Saunders notes, the priestly class of Abijah to which Zechariah belonged (Luke 1:5) "would have been on duty during the second week of the Jewish month Tishri, the week of the Day of Atonement or in our calendar, between Sept. 22 and 30".

"While on duty, the Archangel Gabriel informed Zechariah that he and Elizabeth would have a son (Lk 1:5-24). Thereupon, they conceived John, who after presumably 40 weeks in the womb would have been born at the end of June. For this reason, we celebrate the Nativity of St. John the Baptist on June 24," he writes.

"St. Luke also recorded how the Archangel Gabriel told Mary that Elizabeth was six months pregnant with John (Lk 1:36), which means the Annunciation occurred March 25, as we celebrate. Nine months from March 25, or six months from June 24, renders the birth of Christ at Dec. 25, our Christmas."

Christmas was therefore not originally a pagan mid-winter festival. From its inception it has always been a Christan celebration of the birth of Christ, taking place on the day on which the biblical evidence says that he was born.

However, although it was not originally a pagan-midwinter festival, it can be argued that in the modern secular Western world it has increasingly become so. Indeed, it can be argued, as CS Lewis does, that there are now two concurrent festivals, the non-Christian festival of 'Xmas' and the Christian feast of Christmas.

Lewis makes this point in his 1954 essay 'Xmas and Christmas: A lost chapter from Herodotus.' As its title indicates, this essay is alleged to be a lost chapter from the works of the Ancient Greek historian and geographer Herodotus, who is well known for his accounts of non-Greek nations.

The essay starts by describing the fact that off the coast of Northwest Europe there lies the island of Niatirb (try reversing the order the letters) and that, 'in the middle of winter when fogs and rains most abound,' the inhabitants of this island "have a great festival which they call Exmas, and for fifty days they prepare for it in the fashion I shall describe".

He writes, "First of all, every citizen is obliged to send to each of his friends and relations a square piece of hard paper stamped with a picture, which in their speech is called an Exmas-card."

It then goes on to say that the Niatirbians also send gifts to one another "such things as no man ever bought for himself".

"For the sellers, understanding the custom, put forth all kinds of trumpery, and whatever, being useless and ridiculous, they have been unable to sell throughout the year they now sell as an Exmas gift ... But when the day of the festival comes, then most of the citizens, being exhausted with the Rush, lie in bed till noon. But in the evening they eat five times as much supper as on other days and, crowning themselves with crowns of paper, they become intoxicated. And on the day after Exmas they are very grave, being internally disordered by the supper and the drinking and reckoning how much they have spent on gifts and on the wine."

Sound familiar? However, the essay adds that a few Niatirbians have a separate festival "called Crissmas, which is on the same day as Exmas" on which they do "the opposite to the majority of the Niatirbians, rise early on that day with shining faces and go before sunrise to certain temples where they partake of a sacred feast".

A priest explains that Crissmas and the Crissmas Rush "distract the minds even of the few from sacred things".

"And we indeed are glad that men should make merry at Crissmas ; but in Exmas there is no merriment left."

Finally the essay observes: "But what Hecataeus says, that Exmas and Crissmas are the same, is not credible. For first, the pictures which are stamped on the Christmas cards have nothing to do with the sacred story which the priests tell about Christmas. And secondly, the most part of the Niaturbians, not believing the religion of the few, nevertheless send the gifts and cards and participate in the Rush and drink, wearing paper caps. But is not likely that men, even being barbarians, should suffer so many and great things in honour of a god they do not believe in."

Lewis' essay is obviously a work of satire in the tradition of earlier English satirists such as Jonathan Swift. However, it makes two serious points which are laid out in the extracts just quoted. The first is that, although they occur at the same time, the celebrations of Exmas and Christmas are in fact two distinct things. Secondly, the existence of Exmas distracts even the minds of the faithful from the contemplation of sacred things.

These points by Lewis are, if anything, even more relevant today than they were back in 1954 and what they point to is the theological reality that Exmas has become a festival marked by idolatry.

The first commandment of the Ten Commandments is 'You shall have no other gods before me' (Exodus 20:3). As Martin Luther observes in relation to this commandment: 'That ...upon which you set your heart and put your trust is properly your god.' For most people who celebrate Exmas the gods they have their hearts upon are material possessions, relationships with family and friends, and the copious consumption of food and drink, and the reason they set their hearts upon these things is because they trust that they will give them the happiness which they desire.

However, like all idols, all material things worshipped as gods, the gods of Christmas will ultimately fail to deliver. As Augustine famously puts it at the start of his Confessions, God has made us for himself and our hearts will be restless until they find their rest in him. The gods of Exmas cannot provide us with that rest, that true happiness, in this life, or give us eternal life in the age to come.

Only the God of Christmas, the God whose birth as a human being took place on 25 December, can give us the true happiness we seek in this life and eternal life in the age to come. That is why it is so important that Christians do not let Exmas distract their minds from the celebration of Christmas and why they should seek to remind their non-Christian neighbours about the importance of Christmas, even in the midst of the Exmas celebrations.