"There are hundreds of Greek, Egyptian and Roman myths about babies being born on December 25, why should we believe yours?" The young Goth, dressed in black from head to toe, was insistent. In a packed lecture theatre in Belfast I felt compelled to answer, perhaps not in the wisest manner. "You, sir, are a prime example of the dangers of Wikipedia," I began, before going on to point out that there were not hundreds of myths.

When the young man met me at the end of the talk I apologised for breaking the first rule of public debate – 'Don't belittle your opponents.' But he just laughed and said "No, I thought you were going to give me some of that Christian b******t , and I was going to walk out, but you called me on it. Cheers."

But this young man is not alone. Anyone who has watched Zeitgeist, read Christopher Hitchens or been involved with any of the plethora of new atheist websites will have this issue raised. It is alleged that the Christians just borrowed the myths of the Babylonian Marduk, the Egyptian Horus or the Greek legend of Perseus and came up with the virgin birth of Christ. To the uneducated and those who have a desperate need to debunk Christianity this seems so obvious that it must be true. It just makes so much sense – after all virgin births don't happen and Christianity is just made up anyway.

But on closer examination the whole case just falls apart. Firstly the comparisons are at best tenuous. It is true that in ancient mythology there are numerous tales of various deities impregnating humans or getting pregnant themselves through a variety of means, but there is nothing to compare with the miraculous conception of a child conceived in the womb of a teenage mother by the Holy Spirit of the One God. It is literally incomparable.

Secondly anyone who knows anything about the Jewish faith would realise that the early Christians, who were largely Jews, would not have taken on well-known pagan myths and tried to pass them off as their own. The very idea would have been abhorrent.

Critics like the late Christopher Hitchens of course have no doubt that virgin births cannot happen. They even tell us what the Bible really means. But like most of the New Atheist myths, these are largely bluster. Hitchens' comments on Isaiah 7:14 are particularly interesting – "The word translated as 'virgin', namely 'almah', means only a young woman." Unlike Hitchens, EJ Young and Robert Dick Wilson did serious research on the meaning of the nine occurrences of 'almah' in the Old Testament. Both concluded that the word is never employed to describe a married woman and that the Septuagint (cited by Matthew's Gospel) was right to translate it in Greek as 'parthenos' (virgin).

Hitchens though is in good – or bad – company. There are many more 'sophisticated' church leaders who are stuck in a 19^th-century paradigm of 'miracles don't happen' and so do their best to dismiss the virgin birth as untrue or unimportant. Tony Jordan, a scriptwriter for the BBC series Eastenders, did an excellent series on the nativity. He describes his experience in researching this: "I sat with these men of the cloth, these were organised religion. They were all explaining to me about the nativity and about how it never happened. And they were saying, "Well of course, Mesopotamia... mumble, mumble – there was always the legend of the virgin birth." And I'm thinking, 'What? Hang on a minute! You're on the wrong side, that doesn't work.' So I despair of them" (interview in Christianity Magazine, March 2012). Indeed.

I suspect that for most people the idea of the virgin birth is one that is abhorrent because it seems so impossible. But I have never understood why it is seen as such a stumbling block. If human beings can create a situation whereby a woman can become pregnant without the necessity for sexual intercourse, why should we consider it impossible for an Almighty God to do so? The trouble is that people start off with the pre-supposition that such a God does not exist and therefore a non-existent being cannot perform such a miracle. This is the ultimate in circular and irrational thinking. To claim that a virgin birth cannot happen because the being who could make such a thing happen does not exist, really says nothing, other than about the prejudices of the person making the claim. Likewise, I am not stating that merely claiming it did happen makes it true. However I am stating that by definition it is not self-evidently impossible that an Almighty God could do this one small miracle.

Larry King was once asked who he would like to interview if he had his pick from all history. His answer? Jesus Christ. "What is the one question you would like to ask him?" "I would ask him if he was indeed virgin-born, because the answer to that would define history for me." It really is that important. The 'evangelical' liberal, Rob Bell, likened the virgin birth to one brick in a wall of theology. "What do you lose if you lose that one brick?" To which the best reply was – "Nothing, except Jesus." The virgin birth of Christ is one of the key doctrines of Christianity and without it you do not have Christ. It's a bit like the man who goes into the local fish 'n' chip shop and announces 'I'll have a fish supper, without the fish.' Christianity without the virgin birth of Christ is Christianity without Christ.

For me the truly unbelievable thing is not that God could come to earth, but that he did. And that the child's birth we celebrate on December 25 is the I AM, the Light of the Word, the Word, the Son of God, our Lord and Saviour.

"Veiled in flesh the Godhead see,

Hail the incarnate deity,

Pleased as man with man to dwell,

Jesus our Immanuel".

Christmas without the virgin birth is Christmas without Christ. That does not make it Xmas, it just makes it Mythmas.

This year, as always, I will be celebrating the reality of Christmas, not the delusions of Mythmas. Happy Christmas!

David Robertson is Moderator Designate of the Free Church of Scotland and Director of Solas CPC.