The trial, thought to be the first of its kind in Afghanistan, highlights a struggle between religious conservatives and reformists over what role Islam will take in the country four years after the fall of the Islamic fundamentalist Taleban regime.
Abdul Rahman, 41, was arrested last month after his family accused him of becoming a Christian, the judge, Ansarullah Mawlavezada, said in Kabul yesterday.
During a hearing last Thursday, Mr Rahman is said to have confessed that he converted from Islam to Christianity 16 years ago while working as a medical aid worker for an international Christian group helping Afghan refugees in the Pakistani city of Peshawar.
"We are not against any particular religion in the world. But in Afghanistan, this sort of thing is against the law," the judge said. "It is an attack on Islam."
Mr Mawlavezada said he would rule on the case within two months.
The prosecutor, Abdul Wasi, said he had offered to drop the charges if Mr Rahman converted back to Islam, but he refused.
He went on: "He would have been forgiven if he changed back. But he said he was a Christian and would always remain one. We are Muslims and becoming a Christian is against our laws. He must get the death penalty."
Afghanistan's constitution is based on Shariah law, which states that any Muslim who rejects Islam should be sentenced to death, according to Ahmad Fahim Hakim, the deputy chairman of the state-sponsored Afghanistan Independent Human Rights Commission.
After being an aid worker for four years in Pakistan, Mr Rahman moved to Germany for nine years, his father, Abdul Manan, said outside his home in Kabul.
He returned to Afghanistan in 2002 and tried to gain custody of his two daughters, now aged 13 and 14, who had been living with their grandparents. A custody battle ensued and the matter was taken to the police.
During questioning, it emerged that Rahman was a Christian and was carrying a Bible. He was immediately arrested and charged, his father said.
Afghanistan is a conservative Islamic country. Some 99 percent of its 28 million people are Muslim, the remainder mainly Hindu. A Christian aid worker in Kabul, who spoke on condition of anonymity, said there was no reliable figure for the number of Christians, though it was believed to be in the dozens or low hundreds. He said few admitted their faith because of the fear of retribution and there were no known Afghan churches.
Mr Hakim said the case could be exploited by Muslim conservatives to rally opposition to reformists trying to moderate the ways the religion is practised here. "The reformists are trying to bring about positive changes," he said. "This case could be fertile ground for extremists to manipulate things."
Muslim clerics still hold considerable power in Afghanistan, especially in rural areas where most women wear all-encompassing burkas and are dominated by men.