In the letter, which also mentions that a messenger from Oswiu had been among the companions of the archbishop-elect, the pope apologizes that he has been unable to find a successor to Wighard yet.
Hindu fundamentalist groups have been holding protests to demand that the pope apologize for massacres by Portuguese colonists in the 16th century and that Christian missionaries stop conversions.
It was taken from his prayer at a Mass earlier this month at St. Peter's Basilica in which the pope apologized for the historic sins of the Roman Catholic Church.
Several extremist groups threatened to kill all Christians unless the pope apologized.
"The pope has apologized, and that's enough, so let's calm down," said Hasyim Muzadi, head of Indonesia's largest Islamic organization, Nahdlatul Ulama.
Hoping to heal old wounds, the pope has apologized for past sins committed by Catholics against Orthodox believers, most notably while he was in Athens.
It said the pope should apologize "so as to ease the fears of Muslims who sense they are becoming the target of an orchestrated campaign."
In Greece this month, the pope apologized for crimes committed by Roman Catholics against the Greek Orthodox Church.
Three years ago, the pope apologized for the church's role in the sack of Constantinople.
The pope, he said, apologized as his guest stooped to pick it up.