He is an expert in Islamic Law...

He is an expert in Islamic Law, a guide and proof of God's existence, a model to follow and a leader as the deputy of the imam. He is wali (leader) when he acts as a leader of someone and a marja'a taqlid when he is referred to in religious matters. Wilayat al-faqih refers to the all-comprehensive authority of the jurist (faqih) in the absence of the twelfth Imam.

"Wilayat al-faqih" may include functions like wilayat al-nizam (the authority to manage public order), wilayat al-qada (the authority to administer justice), wilayat al-siyasa (the authority to administer a government and hold political office and wilayat al-tasarruf (authority for the jurist to act in a way that he judges to be the best in the interest of the community). A corollary to the wilayat al-faqih may include the duty to declare a jihad.

It is to be noted that not all mujtahids accept the concept of the all-embracing and comprehensive view of wilayat al-faqih. Marja al-taqlid refers to the role of a mujtahid who is competent to derive juridical rulings from the Quran and hadith literature and is able to arrive at solutions that are not explicitly stated in the sources, by having recourse to the principles as stated in usul al-fiqh.

Such a mujtahid is qualified to issue juridical verdicts (or fatwa) and is a source of reference for followers, who pursue him by doing his taqlid (which literally means to imitate his acts). Thus, it is possible for a person to be a marja al-taqlid without occupying the position of wilayat al-faqih. In addition, the other way around in also accepted, since in the Iranian constitution the leader is not obligatory a marja, and can be a simple mujtahid having leadership and managerial abilities.

Wilayat is a part of marjayat in the culture of the Shia. The great marjas not only guided people with respect to the divine Law, but they also led people in the particular problems of society, for example when judging between people in particular matters and domestic disputes. The reason that the jurisprudent is an authority in matters of the law is because of his specialization in jurisprudence and his power to derive the rules of God from their sources.

The reason aside from the above-mentioned qualities, for a jurisprudent to become a leader depends on his ability to manage society according to the principles and values of Islam.