I understand that the Qur'an does not make any mention of either Shia or Sunni and warns the Muslims against dividing into sects. If that is correct then how come the Islamic Ummah has got divided into these two broad sects?
There are narrations in both Sunni and Shi'i collections that say that this ummah will divide into seventy-three sects, with only one being saved, i.e. 'the jama'a'. This has been interpreted to mean 'the majority' by Sunni scholars, whereas Imam 'Ali (as) has said that 'the jama'a' are those who follow the truth, even if they are few. This means that, even among the Shi'a, not all will necessarily be among the jama'a. Imam al-Baqir (as) has said 'the fitna is inevitable'. The Qur'an talks about previous generations that attacked their prophets and became divided over their message. There are many reasons for divisions to occur, but one key reason seems to have been for the Arabs to overcome their tribal pride and accept Imam 'Ali (as) as the successor. The Jews and Christians also hid parts of their prophets' messages or changed them, because they thought that their worldly position was jeopardised by challenging aspects of these messages. People are often happy to accept the message as long as they can maintain their worldly status and comforts.