The proof, found on a shadowy wall of the leaning tower, is the remnant of a nativity scene, sculpted from wood and plaster in about 780 A.D. that combines Chinese landscape imagery with the reclining figure of the Madonna, according to the British scholar who found it.
The discovery of what would be the oldest Christian site by far in China opens a new window on religious and art history, confirming what had only been known by analyzing ancient texts: that Christian missionaries first came along the Silk Road to China more than a millennium ago.
But the nativity scene also raises a provocative question about Christianity's roots in China and the troubled relationship between the church and the Communist Party today.
China's government equates Christianity with colonialism and is deeply mistrustful of its ambitions. But if the 8th century Tang Dynasty emperor permitted a Christian community to operate within the confines of an important Taoist monastery located near its capital, then Christianity did more than pass briefly through town. It achieved official recognition and legitimacy in China centuries before Western European missionaries arrived en masse.
"The old claims of Christianity as an alien religion will have to be modified," he said. "Although it will have only mild impact at first, it means that contemporary Christianity has a pedigree, whereas before it was nothing more than a running dog of imperialism."
Palmer discovered the site nearly two years ago, but he is only beginning to publicize what he found on the second floor of a pagoda that had been neglected since an earthquake in 1556 blocked access to the interior.
Few Asian and religious history scholars know of Palmer's discovery or feel ready to comment on it, but those who have heard the news are excited, and say a major historical site has been found that adds texture to the past and a layer of intrigue to the present.
While the discovery does not change the known chronology of when Christians first entered China, it calls for a reinterpretation of the relationship because of the close ties apparently forged between Christian Nestorian missionaries from the Middle East and the Tang Dynasty, said Gail Larminaux of the Silk Road project at UNESCO in Paris.
"Sending missionaries through China is one thing," said Larminaux. "To establish a monastery or some kind of Nestorian establishment so near a royal city gives it a completely different dimension."
When Christian missionaries first arrived in 625 A.D., Xian known then as Chang'an was the Tang Dynasty capital, possibly the largest city in the world and a cosmopolitan meeting point for international traders and wanderers at the Eastern terminus of the Silk Road.
Christian scholars knew that Nestorian missionaries, representing an Eastern branch of the church that later faded, were part of the mix in Chang'an because of a descriptive diary-like stone tablet that was found in 1623 and recorded the history of 7th century Christian travelers.
But Palmer and other scholars said their knowledge of the Nestorians was limited because there was no surviving physical evidence of their activities.
Palmer, an author and expert on ancient Chinese texts, hoped to find one of the churches. He was amazed not only to discover a site, but to also identify the remains of a remarkable nativity scene mixing Christian and Chinese imagery in a pagoda on the grounds of a famous Tao monastery.
As he interprets it, a Christian community of scholars studied next to, and interacted with, the leading scholars of Taoism with the emperor's permission.
"They obviously were placed there to have that dialogue," Palmer said. "The extent to which Christians were deliberately put into the very intellectual and spiritual heart of Tang Dynasty religion is astonishing."
"It may have had more impact than we realize, but what happened to it after Tang?" she said.
In fact, the early Christian influence in China apparently ended in 845, soon after the pagoda was built, when the Tang Dynasty turned against religion, destroying Buddhist temples, defrocking monks and perhaps attacking the Christian community. The nativity scene, at some point, was vandalized. The Nestorians and their church of the east also faded away.
Palmer, head of the Manchester, England-based Alliance of Religions and Conservation, picked up the trail of the Nestorians by studying Christian texts of the era that are the subject of a forthcoming book he has written called "The Jesus Sutras."
The next important clue in his search came from an obscure book written in the 1930s by a Chinese professor. The book contained a curious hand-drawn map that referred to a pagoda as Da Qin, which means "from the west," or can refer to the Roman Empire.
The pagoda's location was depicted inaccurately, probably because Japanese spies charting the area in code created the map, Palmer believes. But he recognized another name on the map: Lou Guan Tai, an ancient Taoist monastery where legend says the famous monk Lao Tzu wrote classic work "The Book of the Way of Virtue."