If a person has authority only in one matter his obedience...
If a person has authority only in one matter his obedience would be obligatory only in that matter and one who has absolute authority in all affairs only he would be the person whose obedience is absolute and only he is the Imam. To mention the Messenger and Ulil Amr by only one word ‘obey’ means that the position of the Imam is similar to that of the Prophet.
Rather, just as Prophethood is from Allah (promulgated) through the angel so is really, the Imamate is an appointment through the Prophet and only because of this the observance of the Imam is like obedience of the Prophet. This is why the word ‘obey’ has not been used between ‘Rasool’ and ‘Ulil Amr’. Contrary to it, though the rank of Prophethood is a very high rank, it is not similar to the rank of divinity. The word ‘obey’ between ‘Allah’ and ‘Rasool’ points towards this.
When Allah Himself has joined the obedience of this group with the obedience of Himself and of His Prophet, then doubtlessly a group must have been appointed by them (Allah and the Prophet) whose command (Amr) would be the Amr of Allah and His Prophet so that their obedience may join with the obedience of Allah and His Prophet.
Otherwise it will be construed that the obedience of oppressive and cruel kings like the king of Byzantine will be called (and understood) the obedience of Ulil Amr, which is like obedience of Allah and His messenger. The ugliness of this is not hidden from my wise person. As Shaykh Tabarsi has said: it is not possible that Allah the All-wise can permit the absolute obedience of a person except that of a person whose obedience be proved.
Allah knows that man’s internal condition is just like his outward appearance and about him He may be satisfied that he will neither commit any mistake nor will do any ugly deed and these virtues are not possessed by scholars and rulers except the Infallible Imams.
Allah can never command the obedience of any person who may disobey Him and may order the obedience of a group whose deeds may be different from their words because the obedience of the differing group is impossible just as their agreement on a matter in which they have differed.