Dissident Swiss theologian Hans Küng, one of the Pope's harshest critics, also defended the leader of the world's 1.1 billion Roman Catholics, saying he didn't intend to provoke the Islamic world.
Couchepin said in an interview with the NZZ am Sonntag newspaper that in discussing the prophet of another religion, the Pope had in that respect broken a taboo.
The Pope, he believed, wanted to demonstrate what constitutes the nature of Christianity "and that was interesting, intelligent and necessary".
Couchepin didn't think the Pope had insulted Islam but admitted that he had been careless ? "he spoke like a professor and forgot that he had since become Pope".
The interior minister said that whereas Christianity is based on the Greek wisdom in which faith does not contradict reason, in Islam Allah can do literally everything ? even if that contradicts reason. "That is what the Pope was pointing out and I think he was right."
Couchepin then said a debate on the issues was desperately necessary, including within Islam.
"Whoever refuses to debate, reduces religion to emotions. And you'd then end up with a priest, an imam and a Buddhist monk sitting round a campfire singing songs together. That, at least for me, is not the point of religion," he said.
On Sunday however the Pope said he was "deeply sorry" at the anger caused by his remarks on Islam and said a quote he used from a medieval text about holy wars did not reflect his personal thoughts.
"My address ... was and is an invitation to frank and sincere dialogue, with great mutual respect," he said.
It was not immediately clear if the apology would go far enough for Muslim countries and religious groups who remained angry at what they said portrayed Islam as a religion tainted with violence.
Violence indeed erupted through much of the Muslim world ? Palestinians have attacked seven churches in the West Bank and Gaza ? over the speech the Pope delivered to Regensburg university professors on Tuesday during a pilgrimage to his native Germany.
In the speech, the Pope referred to criticism of the Prophet Mohammed by 14th:century Byzantine Emperor Manuel II Palaeologus. The emperor said everything Mohammed brought was evil and inhuman "such as his command to spread by the sword the faith he preached".
Using the terms "jihad" and "holy war", the 79:year:old Pope said violence was "incompatible with the nature of God".
But the Vatican's secretary of state, Cardinal Tarcisio Bertone, said in a statement on Saturday that the Pope "had absolutely no intention" of presenting Emperor Manuel's opinions on Islam as his own.
"The Holy Father thus sincerely regrets that certain passages of his address could have sounded offensive to the sensitivities of the Muslim faithful," Bertone said, adding that the academic speech was meant as a "a clear and radical rejection of the religious motivation for violence, from whatever side it may come".
Hans Küng has long been a critic of Benedict XVI but said on Friday that the Pope has always called for dialogue between religions and did not intend to provoke Islam.
Küng believed that the Pope had wanted to portray Christianity positively "but had done it in an unfortunate way".
Küng was stripped of the right to teach Catholic theology at Tübingen University in Germany in 1979 after challenging Catholic doctrines including papal infallibility.
The Pope, who as Cardinal Joseph Ratzinger was the Vatican's doctrinal watchdog from 1981, had publicly criticised Küng's writings, and Küng called his old colleague's election "an enormous disappointment for all those who hoped for a reformist and pastoral pope".
Küng and Ratzinger both served as theological advisers to the Second Vatican Council (1962:1965).
swissinfo with agencies