But corruption is an ugly thing.
Verily the merciful Allah is very kind to the servants **. (Sura Baqarah 2:207)** The proof of His being extremely kind and merciful for His servant is that he never allows any shortcoming in the details of acts and deeds. Therefore He has made His servants aware of even little things like depilation and moustache cutting and about cleaning of private parts with stone and water and even about copulating, through His merciful and kind messengers explicitly.
Thus it becomes clear that it is extremely essential for the messenger to appoint his caliph or successor who may protect the laws, rules and regulations of the Shariat after his (messenger’s) departure and who may defend his followers from enemies and opponents and that this is more important and essential than the aforesaid details.
Now when Allah is never slack even in the matter of things of little importance then how can He neglect the most important need of perfecting such great essentiality, which is the greatest pillar of religion? So it must be believed that Allah has appointed such an Imam who is the ruler of all servants. So when He revealed to the Holy Prophet (S) to appoint his successor (Imam) all the Muslims agreed that the Nass was in favor of none but Amirul Momineen (a.s.).
Fourth argument The Ahle Sunnat agree that, according to the Divine rule, Allah has never called back (made to die) any of His messenger, right from Adam (a.s.) to Khatam ( S), without appointing their successor. The same had been the rule of the Holy Prophet (S) too. He had never left for any travel or any battlefield, without first appointing his caliph. Even when he did not go out of town he used to appoint rulers for every place where Muslims lived either in a small numbers or big.
Even while sending someone for a battle he never failed to appoint a chief or leader and this appointment never depended on the liking of the people. He himself used to make such appointment according to the divine command. This was his rule during his every absence, be it a long or a short one.