>>14104
>Do you have to follow an Ayatollah/Marja?
YES. For the vast majority of all Shi'a who follow the Usuli school, it is absolutely obligatory to seek out the most learned of the marja and follow him
However, a Shi'a may follow more than one marja if either his primary marja allows it, if more than one marja is equally the most knowledgeable among all the mujtahids, or if it is recognized that the different maraji possess their own individual fields of expertise that require the layperson or muqallid to consult different experts for each of the different matters that pertain to his predicament. The marja himself may also recommend consulting another marja or mujtahid if he is unable to answer an issue.
There is a small minority of Akbaris, mostly concentrated in some parts of Iraq & Bahrain who do not recognize the authority of any of the maraji at all, but this is because they reject any all independent reasoning/ijtihad and are more solely scripture based.
>Or can you follow the teachings of any Learned Syed like Hossein Nasr instead?
Nasr is not a qualified scholar of law, rather he is an intellectual who is well educated in Islamic history and philosophy.
>Is Hassan Rouhani an Ayatollah?
I'm pretty sure he isnt
>If so, why isn't he on the list of Marja on Wikipedia?
Simple, because he's not a marja.
>What are the types of clerics basically?
On the lowest ends of the spectrum are the khutaba (preachers) and those authorized as sheikhs or mullahs. These are people who have completed a certain amount of time in seminary and are authorized to teach Islam and/or manage the masjids and husayniyas, however they don't have any authority to issue legal rulings and are not mujtahids (those allowed to use ijtihad in legal matters). Among some communities, the ranks of khutaba may be filled by local dervishes, poets or school teachers and such who are simply recognized by the community for their intellectual or moral prowess. The term "mullah" used to just refer to a kind of sheikh, but many Iranian diaspora and critics of the Iranian government use it as a derogatory term for all the Shi'a clergy regardless of rank. When one has reached the rank of sheikh, one is allowed to wear a turban (sayids wear black, non-sayids wear white, though I've read that they used to wear blue years ago)
You then have those who are mujtahids, having received a certificate of ijtihad from a recognized hawza. Among the mujtahids, the rank of "ayatollah" is often given to one who has received authorization not only to follow his own independent reasoning in legal matters, but also has been given authorization to issue legal rulings for others. Once one is a mujtahid, one is no longer obligated to follow a marja, but may choose to follow a marja if he pleases. A marja is an ayatollah of esteemed rank who has developed a sizable following and is recognized by a sufficient number of other mujtahids as among the most knowledgable of mujtahids. A marja can simply emerge by enough laypeople and other ayatollahs themselves simply choosing to follow him that his claim to be a marja-i-taqlid (source of emulation) is relatively uncontested or he can be declared as a qualified marja-i-taqlid by another reputable marja. Following a marja of course, only pertains to matters of the shariah and not to Usul al-Din (Principles of Faith) which not even the marja may go against. Also, the title of "Ayatollah" in reference to a high ranking mujtahid is somewhat recent and has only been around for about 100 or so years, so just because one is not calling himself an "ayatollah" doesn't mean he doesn't still possess the qualifications of one. The term "Hujjat al-Islam" is sometimes used for mujtahids who are almost ayatollahs but not quite.
There is also the ranks of sheikha and mujtahideh, which refer to female sheikhs and female mujtahids. There is a lot of debate on whether a mujtahideh can become marja-i-taqlid. Some say yes, some say no. Most say no.