Some cause for hope:
The pope is the first of some 25 Christian figures addressed specifically in the 29-page letter, titled "A Common Word Between Us and You."
The letter quotes the Koran and the Bible, particularly the New Testament, to illustrate how their basic principles mirror each other. It says the Prophet Muhammad's stance was perhaps inspired by the Bible.
The letter notes that there are differences between the religions and that Islam teaches its faithful to resist those who attack them, but it concludes that the world's two largest faiths should compete only "in righteousness and good works."
Among the 138 signatories were senior theologians from around the world, including Sheik Ali Gomaa, the grand mufti of Egypt and hence the top Sunni Muslim figure there, as well as about a dozen other grand muftis. Ayatollah Mostafa Mohaghegh Damad, a Shiite Muslim from Iran, also signed.