AROUND the besieged city, tens of thousands of knights were pitching tents, watering their horses, lighting campfires. Monks were chanting prayers; priests were giving blessings.
Soon, in the name of God, they would butcher everyone inside the walls.
The year was 1209 and the mighty forces of the Pope had come to stamp out a heresy that had taken hold in southwest France. Inside the city of Beziers, fierce devotees of Catharism - a mystical offshoot of conventional Christianity - were determined to resist.
The Cathars' faith was a complex one which defied the dictates of Rome by denying the divinity of Jesus Christ and rejecting his Resurrection, the very core of Catholic belief.
They also venerated Mary Magdalene - a woman branded a prostitute by the Papacy - as a key figure in the history of the Church. Some even said she and Jesus had been man and wife.
Pope Innocent III had sent his crusaders to crush such dangerous notions.
Their assault began when a mob of foot soldiers rushed a small gate, while others brought up long ladders to scale the walls.
Suddenly, the whole army was pouring in, slashing and stabbing.
When one soldier asked how to pick out heretics from true believers, the abbot in charge of the attack told him: 'Kill them all. God will recognise his own.' In the church of St Mary Magdalene, 1,000 women and children were put to the sword as they prayed.
Blood ran in the streets. Babies were hurled from towers.
The death toll that one morning was close to 20,000, a staggering feat of single-minded butchery in the days before gunpowder made mass murder easy.
The leader of the crusade proudly wrote to the Pope that 'the workings of divine vengeance have been wondrous'.
The campaign against the Cathars lasted a further 30 years, until the last of them had retreated to a mountaintop castle. Cut off and with no hope of escape, the diehards surrendered in 1244 and were marched barefoot to a huge bonfire where they were burned alive.
As far as the Catholic church was concerned, the most serious threat to its power and orthodoxy for close on 1,000 years was consumed by those flames, the ashes of its adherents scattered to the four winds.
How extraordinary, then, that 750 years later, elements of that same 'heresy' should be back in force, attracting new believers and again threatening to undermine orthodox Christianity and its teachings.
This time, it is the Church that feels under siege - its defences chipped away by the runaway success of American writer Dan Brown's bestselling novel, The Da Vinci Code, and the astonishing claims on which it is based.
A popular thriller might seem an unlikely source of theological ferment, but Brown's book is a modern phenomenon. Its publishers claim to have 17 million copies in print worldwide - enough, piled on top of each other, to make a mountain 75 times higher than Everest, or to stretch end-to-end from London to the Vatican and back.
In the past month, up to 80,000 copies were sold in British bookshops every week, and no doubt many of them were unwrapped as presents on Christmas Day.
This is hugely ironic, because the core of the novel - as its author freely admits - is an attack on the very beliefs that Christmas represents.
Christians believe that, in that stable in Bethlehem, the 'Word was made flesh' - that God became Man in the form of Jesus to take on the sins of the world. That is what the whole story of Nativity and the child in the manger means.
Not so, said the Cathars 700 years ago. Not so, says the argument of The Da Vinci Code. According to the book, Jesus was just a man, gifted and great, but totally human - his divinity a fiction fostered by the Church to bolster its own authority.
What's more, this all-too-human Christ not only married the supposedly 'fallen' Mary Magdalene but also sired a daughter by her, establishing a royal bloodline that endures to this day, with the couple's descendants still among us. …