Imam, the successor of the holy prophet and the one who perpetuates the religion and accomplishes the humans' needs, is a transcendental figure with specific characteristics in support of his role as the leader and guide to the people. These include:
• Piety; he should be infallible, i.e. committing no sin, mistake, or error.
• Knowledge; his Knowledge is the continuation of the holy prophet's inspired knowledge which comes directly from Allah and therefore trustworthy to accomplish all materialistic, spiritual, religious and earthly needs of people.
• Virtue; he should be adorned with the best of virtues and sayings at the highest degree.
• Management; he should be able to manage and control the whole human society according to Islamic teachings.
Considering these characteristics, we come to the conclusion that electing Imams is out of the human capabilities and just Allah, the Omniscient, is capable of assigning an Imam as the successor of the holy prophet. Thus, one of the most important characteristics of Imam is to be appointed by Allah. For these are of high importance, let's have a review on them:

Knowledge:
Since Imam is the leader of people, he should know all aspects of religion, be aware of the rules ( i.e. Shariah ), know the interpretation of the whole Koran, be conversant with the holy prophet's Sunnah ( his rituals and teachings ) in order to explain teachings of Islam, answer all questions in all fields and guide the people. Here is no question that such a knowledgeable source is trustworthy and authentic for everybody and such knowledge can be accepted just with a link to Allah's omniscience as Shiites believe that true successors of the holy prophet attain their knowledge from the boundless knowledge of Allah.
Imam Ali (P.b.u.h) introduces a real Imam as:
"Imam is the most knowledgeable person about permitted things and forbidden ones, various rules and Allah's "do's and do nots", His commandments and prohibitions and everything people need."

Infallibility:
The primarily important feature for Imam is infallibility which has its root in the strong will power and divine knowledge that Imam holds and prevents him from committing any sort of sin, mistake or error. This infallibility includes his understanding and elaboration of Islamic teachings as well as fulfilling them and recognizing the expedience of the society.
Some logical and narrative reasons exist, supporting the infallibility of Imams. The most important logical reasons can be defined as:
a. Being able to save the religion and its rules and rituals, Imam should be pure and infallible. Since he is in charge of guiding and leading people to the right path of Islam and keeping the religion away from deviations and heresy, he can highly affect the whole society through his deeds, words and confirmation or disconfirmation of others' deeds. Thus, he should be infallible to guide the people correctly.
b. As mentioned above people sin and make mistakes and therefore they cannot find the true meanings of religion. Now if their leader is the same, how can he be trustworthy? In other words people do not rely on someone who makes mistakes too.
There are some other reasons based on Koran, one of which is 2/124. In sura Baqareh (The Cow), verse 124 it has been mentioned that Allah granted to Abraham (p.b.u.h) Imamate after his prophet hood. Then he asked Allah to run this legacy in his descendants as well. The response was:
"My covenant does not embrace the ones who would
Transgress"
This means those cruel descendants of Abraham (p.b.u.h) would not wear the gown of Imamate.
Considering the verses which emphasize polytheism as the greatest cruelty and any violation of divine orders as oppression to one's self, it can be concluded that whoever committing whatever violation to their selves are as cruel and violator ones and therefore do not deserve the position of Imamate.
In other words, for sure Abraham (p.b.u.h) did not ask Allah to put his sinful descendants or those who were of good deeds but sinful now on the throne of Imamate. Thus here remain two groups:
1. Those that were sinful but asked Allah for forgiveness and carry out Benevolence afterward.
2. Those who committed no sin at all.
And according to the above verse Allah had excluded the former and therefore Imamate can be attributed to the latter.

Social Management:
Human beings are social creature, whom the society affects his thought and behavior a lot. Thus, in order to be well-trained and gain divine favor, he needs appropriate social prerequisites, which is achievable just through the formation of a divine government or a theocracy. In this way Imam should be able to manage the society in accordance with Koran and Sunnah of the holy prophet and use efficient individuals to establish an Islamic government.

Virtue:
As the leader of Islamic society, Imam should be away from all meanness and rascality and be endowed with all virtues and perfections at the highest degree to be a good model to be followed for other people.
As Imam Reza (p.b.u.h) says:
“Imam holds some features: he is the wisest, bravest,…, the most Virtuous, patient, generous, and pious of all people."
However, he should care about training of people, for he is the holy prophet's successor. And it means that he should be adorned with the virtues more than anyone else.
Imam Ali (p.b.u.h) says:
"He, who is appointed as Imam, should take care of his Own training and purification more than others and try to train others with his deeds and behavior."

Being Appointed by Allah:
According to Shiite tradition, Imam and the prophet's successor can be appointed just by Allah and the prophet is allowed to introduce him to people afterward. Therefore, no one is allowed to interfere in this subject.
Here some reasons for this divine appointing can be counted:
1. According to Koran, Allah is Omnipotent and Omnificent and thus all should obey Him. There is no doubt that He can assign His rule over the people to whoever He wishes – based on his competence and His expediency. Hence Imam is like the prophets appointed by Allah.
2. As we mentioned before, Imam should hold some characteristics as infallibility, knowledge, etc. and with no doubt just Allah, the Knower of the invisible and visible, can find such a person with the highest level of perfection. As He says to Abraham (p.b.u.h):
" I am making you for people an Imam"

Last word: a complete and delightful definition of Imam:
To complete the subject, let us finish this chapter with a delightful Hadith, quoted from Imam Reza (p.b.u.h) about Imam and his characteristics:
" They – who argued about Imamate and thought he can be chosen by people's wish – were fools … . People absolutely do not know the position Imam holds among the nation. So how can they choose him to their wish?
Imamate ,surely, is of the highest value, place and dignity, and more like a deep ocean which people cannot search its reaches with their reasoning… .
Allah assigned Imamate as the third and highest level of perfection after 'Prophet Hood' and 'being His own friend' to Abraham (p.b.u.h) … [In fact] Imamate is the succession of Allah, His prophet Mohammad, and the position which Ali (p.b.u.h) and his sons Hassan and Hussein (p.b.u.h) took. Truly, it is the controller and leader of the society, which leads the faithful ones to the path of salvation and honor. The religious duties, as prayer, fast, Hajj, Jihad, protecting the borders, etc. flourish just because of his presence [or accepting his sanctity].
Imam can make halal (allowed things), forbidden or vice versa [and his judgment is Allah’s]; he punishes the sinfuls according to Allah’s rules, sets up the religious rules and invites men to Allah through valid reasoning and fluent preaches.
Imam lightens the whole world like a rising sun but stands unattainable in the horizon. Imam is the shining moon, light of guidance and the pole star [that shows the way] in the dark days of being lost or in the stormy sea, vast deserts, and endless roads [ and he saves us from all ignorance and wickedness]…
Imam is a true companion, a kind father, a real brother, a compassionate mother to her little child and the supporter of men in difficulties and hardship. He is infallible and away from any mistake or error. He holds the whole science and carries the sign of patience and self – control … He is unique at his own time and there is a far distance between his knowledge and others’. No scientist or philosopher can vie with him and there is no one equal to him.
Hence, who can reckon Imam or has the ability to appoint him? Alas! Alas! Here the reasons are strayed, minds are confused and the eyes are blind to see him; the nobles are humiliated and the philosophers are mixed up. No eloquent writer can describe his greatness and virtues and all confess their weakness.