What can a hereditary monarch and a revolutionary leader have in common? If both are Muslims, they can share a view that religion should be the guiding principle of earthly rule.
And what can most divide the hereditary monarch and the revolutionary leader? Religion, too -- if they back varying interpretations of Islam.
This paradox is behind a recent interchange of letters between Ayatollah Khomeini and Saudi Arabia's King Khalid.
The trouble started, as often happens, during this year's hajj (pilgrimage) to Mecca. As King Khalid wrote, "Some of the Iranian pilgrims gathered behind the Black Stone and one of their elders began shouting slogans."
The King insisted that the Saudi government treated the Iranians "reasonably and leniently." Independent reports reaching Beirut, however, speak of at least four being killed in clashes between Iranians and the police.
King Khalid signed his letter with a veiled barb: "We ask God to grant success to all who act in the interests of Islam and the Muslims."
Khomeini's reply underlined his own, more revolutionary, view that the hajj should be a center of political as well as religious mobilization:
"If the hijaz government made political and Islamic use of this religio-political ceremony which takes place every year ... then it would have no need of America, its AWACS aircraft, or other superpowers," he wrote.
(His reference to the "hijaz" government, using a term meaning the area containing Mecca, implies nonrecognition of the Saud family's mandate over the area.)
The Saudis must have been in a difficult position. There probably was resentment in Mecca against the Iranians, who are Shiite fundamentalists, among the ranks of the Sunni Muslims, who form the bulk of the body of pilgrims.
But fundamentalism is on the upswing among Sunnites, too. The Saudi rulers have to keep the balance somehow. But that must include not compromising any tenets of Islam -- and that includes the return of east Jerusalem to Islamic jurisdiction.