The Prophet (peace be upon him and his progeny) said:
لِكُلِّ اُمَّة صَديقٌ وَ فارُوقٌ وَ صَدِيقُ هذِهِ الاْمَّةِ وَ فارُوقُها عَلِىُّ ابْنُ أَبِى طالِب(ع).
Every nation has a man who is truthful and esteemed, and the one who is truthful and a distinguished from my nation is ‘Ali Ibn Abi Taleb. 1
For completion of the program of a genuine religion, an everlasting religion like Islam, there should be someone familiar with all aspects and details of that religion and be able to distinguish between truth and falsehood (deserving the name of distinguished).
Islam needed such a man after the demise of the Prophet (peace be upon him and his progeny), the most part of whose mission was spent in different conflicts with obstinate enemies. That person had to be someone very truthful, honest, eloquent and frank so that he could remove any ambiguities and doubts that crop up among people in the absence of the Prophet (peace be upon him and his progeny).
This rank, as per the above explicit statement, was allocated to Imam ‘Ali (a.s.).