The earliest polemic against Christianity focused on the circumstances of Jesus’s birth. “We have not been born of fornication,” says a hostile gathering to Jesus in John’s gospel. The implication being: we weren’t, but you were. In the second century, the Greek writer Celsus wrote a book about how Jesus was the illegitimate low-birth offspring of a spinner called Mary and a Roman soldier called Panthera. The implication may also have been that she was raped. Various later rabbinic texts refer to him as Jesus ben Pandera. All of which was intended as an insult: Jesus was a bastard. Obviously the son of God couldn’t be a bastard. So, the argument goes, Jesus was not the son of God.

Most scholars agree that 'virgin' is probably a mistranslation of the Hebrew word for young woman

The idea that Jesus was born of “pure virgin” could well have been a reaction to these insults. That Mary’s womb was “spotless” was perhaps a cover story designed by Jesus’s supporters to explain a more God-like nature for his arrival. But here’s the problem. For what separates Christianity from other religious traditions is that – the birth narratives aside – Christianity deliberately refuses the familiar distinction between the pure and the impure. Jesus was born in a cowshed; from lepers to prostitutes, he deliberately courted the ritually unclean; and he spent most of his ministry tearing down barriers between pure and impure – not least, those of the Temple – that separated the “ungodly” from God himself.

In Christianity, purity is abolished. Indeed, the core idea that the all-perfect God almighty might actually steep so low as to be born as a bleeding, defecating human being would have been regarded by all previously orthodox believers – both Greek and Jew – as disgusting. But this is the central insight of Christianity: that in the person of Jesus, there is no contradiction between being fully human and fully divine. Or, in other words, God is perfectly at home in a human life, with all its ritualistic mess, from blood to semen. There is no shame in the constituent elements of our humanity, including the manner in which we are made. Which is why the “pure virgin” tradition runs totally against the grain. The problem is not just basic biology: it doesn’t add up theologically.

That the cult of Mary’s virginity came to play such a large role in the development of Christianity is a testament to the powerful hold that purity has on the religious imagination. From this perspective, her womb became the new Temple, a pure place for God to reside. But this damaged the basics of Christian philosophy by persuading too many that sex is dirty and that women ought to model the impossible combination of virgin and mother. Outside the first two chapters of Matthew and Luke, the virgin birth plays no further part in the New Testament. St Paul makes no mention of it. And most scholars agree that virgin is probably a mistranslation of the Hebrew word for young woman.

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Furthermore, Jesus didn’t much care for the whole nuclear family thing, insisting that the most important paternity one could claim comes from God, not from one’s biological parents. Hence the Christian requirement to be born a second time – the water of baptism being more existentially prevailing than the blood of family. This, as St Paul puts it, is a metaphorical rebirth by adoption into a new, non-biological family. Which is why, to my mind, the best Christmas gatherings reflect this – when those seated around the table are, as well as blood relatives, an assortment of “aunties” and “uncles”, adopted as part of an extended family. Or, even more radically, when Christmas lunch is cooked with strangers; the lost, the lonely and the homeless.

None of which is to disparage Mary one bit. A spirited young woman from Galilee, pregnant and unmarried, she sung about how God would pull down the mighty from their thrones and lift up the lowly. It doesn’t matter where you come from or who your parents are. Early Christians answered the likes of Celsus in the wrong way. When they charged Jesus with being illegitimate, they should simply have replied: “So what if he was?”