Two Berlin cabinet ministers expressed their deep concern and Germany's top Catholic cardinal demanded freedom for Abdur Rahman.
On Sunday, an Afghan judge said that Rahman had been jailed for converting from Islam to Christianity and could face the death penalty if he refused to convert back to Islam again. Islamic Sharia law stipulates death for apostasy.
Italy called in the Afghan ambassador in Rome and its former President Francesco Cossiga suggested withdrawing Italian troops unless the man is spared.
With some 2,700 German and 1,775 Italian soldiers in Afghanistan, the development presents a dilemma for President Hamid Karzai, a key ally in the US-led war on terrorism. He needs a foreign troop presence in his country to defend against al Qaeda and Taliban remnants.
"We will do everything possible to save the life of Abdul Rahman," German Development Minister Heidemarie Wieczorek-Zeul told the daily Bild. The newspaper said Rahman had converted to Christianity while he lived in Germany for nine years.
Wieczorek-Zeul said she would make a direct appeal to Karzai.
Foreign Minister Frank-Walter Steinmeier expressed "deep concern" and said he would also intervene if necessary. Berlin's embassy in Kabul was "in close contact with the Afghan side" on this, he told the Frankfurter Rundschau newspaper.
Religious voices weighed in as well. Germany's top Catholic prelate, Cardinal Karl Lehmann, described the case against Rahman as "an alarming signal."
"The German Bishops' Conference solemnly demands that Christians in Afghanistan be able to practice their faith openly and freely and that conversion to Christianity must be possible without any disadvantages," he said in a statement.
"German bishops will try to ensure Christians in Islamic countries enjoy the same rights as Muslims have in our country."
Christians have few rights in some Islamic countries, notably Saudi Arabia and Pakistan. This has become an increasingly sore point in Europe, where Muslim communities are growing rapidly and demanding rights and respect.
Human rights and fundamental freedoms
A statement released by the Italian Foreign Ministry said: "Italy will move at the highest level ... to prevent something which is incompatible with the defense of human rights and fundamental freedoms."
Former President Cossiga wrote an open letter to Prime Minister Silvio Berlusconi and urged him to withdraw Italian troops from Afghanistan unless Kabul ensures Rahman's safety.
"It is not acceptable that our soldiers should put themselves at risk or even sacrifice their lives for a fundamentalist, illiberal regime," Cossiga wrote.
Afghanistan is a conservative Islamic country and 99 per cent of its more than 25 million people are Muslim. A court sentenced two Afghan journalists to death for blasphemy three years ago but they escaped and sought asylum abroad.