Muslims have advanced various parameters and qualifications for leadership of the nation after the Prophet (s.a.w.a.).
These are some of the qualifications cited by various groups for Imamat / Caliphate
companionship of Prophet (s.a.w.a.)
seniority in age
council formed specifically for the purpose (Shura)
However, all these parameters were short-sighted and at best sought to resolve the issue of leadership only for a couple of generations after the Prophet (s.a.w.a.).
They fail to define that one parameter that stands the test of time and can be a guiding light for generation after generation till the Day of Resurrection, since that is how long religion will last.
If only the Muslims had paid close attention to the Noble Quran, they would have learnt of the all-important parameter of Imamat till the Day of Resurrection.
Imam Ali Reza (a.s.) on qualification for Imamat
In a lengthy narration on Imamat, Imam Ali Ibn Moosa al-Reza (a.s.) outlines the all-important parameter for Imamat according to the Noble Quran:
Allah informed (Ibrahim (a.s.)) – ‘Surely I will make you an Imam of men…’
On this, Ibrahim (a.s.) said elatedly – ‘And of my offspring?’
Allah, the Almighty said – ‘My covenant does not include the unjust’
Then this verse nullifies the Imamat for every unjust one till the Day of Resurrection and it is (only) for the chosen ones…
Al-Kafi v 1 p 199-203, 489-502
Tuhaf al-Uqool p 436-442
Al-Ghaibah of Nomani (r.a.) p 217-228
Al-Amaali of Shaikh Sadooq (r.a.) p 673-680
Oyoon al-Akhbaar v 1 p 217-222
Kamal-al-Deen v 2 p 675-681
Al-Ehtejaaj v 2 p 433-437
Clearly, the infallible sons from the progeny of Ameerul Momineen (a.s.) are the Imams of the nation. They are the only ones qualified for the position based on the infallibility parameter defined by the Holy Quran. All other man-made parameters like companionship, seniority in age, selection by consensus, decision of council are short-term, short-sighted and imperfect; they fail to give clear answers on leadership of the Muslims till the Day of Resurrection.