Imamate is the actual dividing line, and describing Shia Islam as Sunni Islam plus mourning for Husayn gets it exactly backwards. For Twelvers the Imam is not merely a ruler but a divinely appointed, sinless guide who carries the authoritative interpretation of the Quran — which means Shia law, hadith, theology and prayer all rest on a different foundation, not merely a different opinion. Sunni Islam holds that prophecy ended and left the community, its scholars, and its consensus in charge, and that the caliphate was a practical office that could be well or badly filled. Zaydi Shia hold a lighter version: the Imam must be a qualified descendant who claims the office openly. Ibadi Muslims, mostly in Oman, reject the whole framework and elect their imam. The Amman Message of 2005 got scholars from all of these to sign a document recognizing each other as Muslims, which was not a small thing.