If a medieval pilgrim was looking for the most extraordinary collection of festive relics – a fragment from the manger, perhaps, or a finger bone of St Nicholas – then the place to go was an abbey built to commemorate one of England’s fiercest battles.

English Heritage has revealed new research into its property Battle Abbey in East Sussex. Today it is an atmospheric ruin but it was once one of the most abundantly endowed religious sites in England.

A medieval manuscript listing its relics has been analysed and transcribed for the first time by English Heritage historian Michael Carter.

It reveals that the relics were the most prestigious given to any abbey, more significant even than those at Westminster Abbey.

The manuscript lists 175 individual relics, many given by the abbey’s founder, William the Conqueror, to atone for the bloodshed at the Battle of Hastings in 1066.

Many are directly associated with the story of Christmas, good and bad. They include objects purporting to be from the ground, swaddling and manger where Jesus was born and wood from the cross on which he was crucified.

There is also a rock used to stone St Stephen, whose feast day is Boxing Day; and bones of several of the Holy Innocents killed on the orders of King Herod, a massacre commemorated in the west on 28 December.

St Nicholas, the original Santa Claus, is represented by a finger bone and fragments from his napkin and hair-shirt.

Carter recalled seeing the manuscript, written in Latin and dating from the early 15th century, for the first time. “I just couldn’t believe what I had come across,” he said. “It wasn’t a low hanging fruit kind of find, it was fruit lying on the floor which everyone before me had overlooked.

The medieval manuscript analysed by English Heritage’s Michael Carter which listed the relics in Battle Abbey. Photograph: The Huntington Library

“It’s fascinating how connections to our Christmas today can be traced back almost a thousand years and despite Henry VIII’s violent suppression, these great monasteries are still giving up their secrets.”

The extensive gifts from the Norman king to Battle are especially interesting because he gave very little to other abbeys. “William the Conqueror was really mean to English monasteries,” said Carter.

The reason was salvation, Carter said. “The bloodshed at the Battle of Hastings, even by medieval standards, was absolutely horrific. The conqueror knew that unless he made atonement for this and served the penances imposed on him, he was going to go to hell.”

The manuscript also reveals another royal connection, with generous donations from King John. In 1200 he gave a relic of the Holy Sepulchre (Christ’s tomb) and a portion of the True Cross, both collected by his brother, Richard the Lionheart, when he was on his crusade in the Holy Land.

All the relics are long gone. All that remains is the newly transcribed document which is in the collection of the Huntington library in California.

Today, medieval relics are mostly viewed with scepticism, few people really believe that a bone from a saint actually came from a saint.

Carter said: “Whenever you mention relics to people, a narrative established at the time of the Reformation, I think, still holds true to this day is that people think ‘God, weren’t medieval people stupid?’

“No they weren’t. They had a different standard of evidence. To the medieval mind a lot of these relics would have been completely convincing.”

Carter’s research is published in the December edition of the Journal of Medieval Monastic Studies.

• The headline of this article was amended on 18 December 2019 to more accurately reflect the text of the article.