It may be going too far to say the Iran-Iraq war started because of a Saudi-Iranian religious dispute.
Saddam started the war by attempting to seize the Arab regions of Iran (which, in Abadan, conveniently contain substantial portions of the Iranian oil industry). His announcement of the conflict justified it in pan-Arab terms, announcing he was going to free "Arabistan" and the three islands (Abu Musa, Greater and Leser Tunb) currently disputed between Iran and the UAE. It was clearly an effort to mobilize pan-Arab support against a formerly hegemonic, pro-Western Iran now wracked by internal disorder. And a blatant stroke of opportunism to make up for the humiliating concessions of 1975. Under Khomeini, Iran had also attempted to stir unrest in the Shi'ite regions of Iraq. And, of course, Saddam was a megalomaniac.
So there may be non-Saudi reasons the war began.
Of course, that's just the opinion of a jaded American whose pay should be cut, according to the master of spin.

Tim