Imamat 1

"When his lord tried Ibrahim with certain words he fulfilled them .He said: surely I will make you an Imam of men. Ibrahim said: and of my offspring? My covenant does not include the unjust, said He."
2:124 Holy Quran

Ibrahim (PBUH) was a prophet , despite of his state as Khalilullah he was tested and tried by Allah (SWT), it would be meaningless if one thinks that Ibrahim PBUH with such high moral and ethics was tested in very elementary matters of his life for the sake of test only such as many other human beings,why then he was tested?! what was the nature of his test? Ibrahim PBUH 's trial was a test to show his steadfastness of his faith in Allah. Allah tried him then made covenant with him , afterward He became Imam :

" Surely I will make you an Imam of men"

this verse guides us to understand Risalat and Imamat better, it indicates that Imamat holds definitely high order which is bestowed by Allah to one who proves himself suitable and worthy after going through a series of tests.
No one however vitreous or godly he might be could never claim the divine position of imamat on his own.
Ibrahim became an Imam for mankind yet he had no right to pass it to his descendants. He requested Allah let children inherit the position , Allah told him : " My covenant does not include the zalim(Unjust), unjust according to quran is the one who commits shirk and worships other than Allah . Allah clearly says that Imamat will never be given to those who have sinned, only infallible one can be an Imam.

My covenant does not include the unjust"

Among the companions of the prophet ,there was no one who has never worshiped idols , except Ali ibn Abi Talib , the prophet cousin, hence the prophet PBUH hand over the imamt covenant to Ali ibn Abi taleb under the command of Allah, and after Ali (as) to the eleven imams in the progeny of Muhammad(PBUH) and Ali (as). Imam must be free from any kind of sins , small or great , hidden or open.
Through the last messenger of Allah the perfect and completed religion of Allah was conveyed to the people, Although the legislation act was ended with the mission of the prophet Muhammad PBUH , but Allah never left this umma alone without genuine leaders.
Ibrahim became imam for mankind , he was not apolitical leader or governing ruler yet people were appointed to obey him in every single matter, spiritual or earthly , nonetheless an Imam doesn't need to be a political face or a temporal power, people have been ordered to obey the imam of their age .

prophet Muhammad PBUH said:
whoever dies not knowing imam of his age , he dies in ignorance