We believe that a fully qualified mujtahid is a representative of the Imam, in the case of the latter's absence. Thus he is an authority over Muslims and he performs the functions of the Imam as regard judgement and administration among the people. Because of this, Imam Ja'far as-Sadiq said:
To deny the authority of mujtahid is to deny the authority Imam, and to deny the authority of the Imam is to make an objection to the authority of Allah, and this is tantamount to polytheism (shirk).
Therefore the qualified mujtahid is not one who issues fatwas, but he also has general authority over Muslims who must consult him if they require judgement, this being obtainable only from him. It is correspondingly wrong for anyone to give judgement except him or one who is appointed by him, as no-one can pass sentence without his permission. Also, all that which belongs to the Imam should be given to the mujtahid.
Such authority has been bestowed upon the qualified mujtahid by the Imam so that he may represent him in his absence ; hence he is known as the representative of the Imam (na' ib al-imam).
