From what I have discovered so far, the split happened right after Muhammad died and the question of succession arose. The Shiites thought Muhammad chose his son-in-law (and cousin) but the Sunni sect thought Muhammad told them to chose someone from among their peers/elders. The new leader became AlouBeccer (sp?) and he said something to the effect that if you worship Muhammad he is dead, if you worship God he lives forever.
In relation to Christian religion, Christians also believe that God lives forever, even after all the prophets, except Jesus, died. (or is this incorrect?)
I sincerely apologize for my possible mis-quotations of the Koran or the Bible but I hope to learn from the intelligence of those around me, irrespective of their professed religions.