Rahman, who is believed to be 41, was charged with rejecting Islam when his trial started last week, the judge said.
During the hearing, the defendant allegedly confessed that he converted from Islam to Christianity 16 years ago when he was 25 and working as a medical aid worker for Afghan refugees in neighboring Pakistan, Mawlavezada said.
Afghanistan's constitution is based on Shariah law, which states that any Muslim who rejects their religion should be sentenced to death.
"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. ... The prosecutor is asking for the death penalty."