Inside, the church was packed and stifling. Here, a woman knelt, her hands clasped in prayer. There, another, swathed in the all-covering black Islamic chador, called on Allah, her palms outstretched to heaven. Christians and Muslims were praying together, side by side. They were praying for a miracle - and they expected to get one.
You can hardly blame the people of Istanbul for wanting a miracle after the devastating earthquake that ripped through their lives and took 15,000 from them last month. But it came as more of a surprise to learn that one happens here every year. Tucked away in a forgotten corner of this ancient city, there is an Armenian church where, annually, Christians and Muslims gather together. They believe that a miracle happens on the same night each year. Someone is cured of an illness or disability.
They had come in their hundreds. People had trekked here from rural Anatolia, and were spreading their bedding on the church floor. They planned to sleep here all night, waiting for God to touch one of them.
To find the church I had to wander through the warren of tiny backstreets that is Balat. This was once the Jewish quarter of the city, and you can still see the Hebrew inscriptions over the doors of long-abandoned synagogues. Now it is a grindingly poor district, haunted by crowds of underage prostitutes.
It was the animals that announced the church. The narrow streets around the grand 19th-century building had become an impromptu market for sacrificial victims. To my amazement, some of those buying were Christians. Sacrifice is an accepted part of Muslim worship in the Middle East, but not Christian.
Aras Ekim held a chicken struggling unsuccessfully under her arm. An ethnic Armenian, she had come all the way from Australia just for tonight. When I asked her whether she was going to sacrifice the chicken, she looked at me as if I she thought I was a halfwit. "Of course," she said.
The priest brandishing his gold crucifix was more forthcoming. "There is only one sacrifice for Christians," he said. "Jesus's death on the cross." So why was he blessing these animals? "Because these people have brought them for the sick and needy," he barked at me angrily.
There were plenty of those. Ader Saykin has been in a wheelchair since he was injured in a boat accident in 1993. He sat in the aisle of the church, hope in his eyes. He is a Muslim. Why had he come to a Christian church in search of a miracle? "I am only praying to God," he said. "Church or mosque, it is not important: everybody rises for God."
Just in front of him sat Makre Dumanoglu, an Armen-ian Christian, like him in a wheelchair. She is only 23: she is in a wheelchair because she was given the wrong injection by mistake. "I came to get better," she said simply. "I think it's special that the Muslims are here: everybody is praying together."
Turkish Muslims have always had an ambiguous attitude to Christianity. Often, you can see them in one of Istanbul's many churches, coming, sometimes furtively, for holy water or a priest's blessing. But I had never seen the faiths praying together before.
"Muslim people pray to God more than we do," said Cigdem Kizilay, a friend who had come to pray for Ms Dumanoglu. "I don't know why, but their belief is stronger than ours."
Just then pandemonium broke out on our left. An elderly woman had fainted. But the people around her were not calling for help. She had gone into some sort of trance, and was calling names. People shouted the names out loud, and a choir began to sing. This apparently was what happened before the miracle. Someone went into a trance and called out the name of the person who would be cured.
But no one came forward, and the woman fell silent. It was one o'clock in the morning. Disappointed, I slipped out of the stifling heat for some fresh air. "It's great to see Christians and Muslims together," said the bemused policeman at the door. "But I don't believe there will be a miracle."
Of course, it happened while I was outside. When I got back, everyone was rejoicing as the choir thundered a hymn of praise. Another woman had fallen into a trance and called the name Velican. Eleven-year-old Velican Tuncer had started speaking. His mother told me he had never been able to speak before. The same had happened to a second child whose family wanted no publicity. Both were Muslims.
A friend turned to me as we left, and said: "It was enough of a miracle to see two faiths that have fought each other so much praying together."