Shias believe that Imamah is of the Principles of Faith (Usul al-Din).As the verse 4:165 of quran expresses the necessity to the appointment of the prophets; so after the demise of the prophet who will play the role of the prophet; till the people have not any plea against <answer>Allah</answer>.So the same logic that necessitated the assignment of prophets also is applied for Imamah.That is <answer>Allah</answer> Must assign someone similar to prophet in his attributes and Ismah as his successor to guide the people without any deviation in religion. They refer to the verse (...This day I have perfected for you your religion and completed My favor upon you and have approved for you Islam as religion...) 5:3 of Quran which was revealed to the prophet when he appointed Ali as his successor at the day of Ghadir Khumm.
Who must assign a successor after the demise of a prophet?
Allah