Infallibility is a spiritual power and inner virtue that restrains one from even imagining and thinking of sinning, let alone committing sin and in Islamic terms, means for one to be protected from sin, mistake and forgetfulness.
Infallibility can be divided into two general groups of: 1- Absolute infallibility 2- Proportional infallibility, which can get different in strength and weakness.
According to this categorization, five groups of people can be called infallible:
1- All of Allah’s (swt) prophets
2- All of the imams
3- All of Allah’s (swt) angels
4- Those other than the abovementioned (because being an infallible doesn’t necessarily mean that one is a prophet or imam) such as Lady Fatimah (as) or Lady Maryam (as).
5- A group of ordinary purified people other than those mentioned above.
Infallibility literally means to be protected and in Islamic terms means for one to be protected from mistake, sin and forgetfulness. In other words, infallibility is a power within that restrains its possessor from even thinking of sinning, let alone committing it.
Being protected from mistake and forgetfulness is of two types:
1- Absolute protection; meaning that one is protected from sin, mistake and forgetfulness from birth till death when it comes to learning and teaching Allah’s (swt) religion. This type of infallibility can only be found in the angels, prophets and imams and there is no reason to say that it can be found in anyone else.
2- Proportional infallibility; which can get stronger and weaker depending on the individual. This type of infallibility can be obtained through effort, self-restraint, and spending nights awake in worship and is also maintained in the same way it is obtained, but the disclaimer is that once obtained, there is no guarantee that it will remain for the rest of the person’s life. Hence, this type is of infallibility can be related to others and is usually referred to as having the virtue of “justice”, meaning that the person has reached a level in which he/she no longer commits the great sins or sins on purpose.
According to this definition of infallibility, not only will the prophets, imams and angels be infallible, but normal people who have reached high levels of purity through worship and self-restraint can also be considered infallible.
Therefore, we will categorize the infallibles into five groups along with the reason for the infallibility of each one:
1- The prophets:
The messengers of Allah (swt) must be free of all sin, small or big. According to Shia scholars, four instances of infallibility are common among all prophets; in receiving and delivering divine revelation, in their beliefs, from sinning, from mistake and forgetfulness.
Rational reasoning for the infallibility of the prophets:
Islamic scholars in the field of theology, have listed many reasons for why the prophets must be infallible. Some of those reasons are as follows:
a) Inner causes: The causes of sin (eg. ignorance and the prevalence of desires and lust) are all under the control and defeated by those that restrain from sin.
b) The reason of “reliance” and “defeating the purpose”: Clearly, the whole goal of sending the prophets is for them to guide mankind through Allah’s (swt) teachings. This goal can only be accomplished if there is no doubt left for the laity regarding their sayings and teachings, such that they consider what the prophets say, exactly what Allah (swt) has said, so that they can whole-heartedly accept what they are saying and completely submit to and have full trust in it.
Naturally, the wise and intellectual one never does anything that is in contradiction with his/her goals and defeats their purpose. If so, people will no longer trust and listen to their prophets and will go astray, which is in complete contrast with what Allah (swt) truly has intended by sending His prophets.
c) Misleading and encouraging to sin is impossible (by Allah (swt)): Indisputably, when guiding His servants, Allah (swt) will never do the smallest thing that might encourage them to commit sins. If He ever supplies someone with proof of being a prophet (eg. miracles) that isn’t infallible (commits sins, mistakes and forgets things), that person will mislead the others instead of guiding them and such a thing is impossible to take place on Allah’s (swt) behalf.
d) The unworthiness of one who isn’t infallible to receive revelation:
We all know that every important mission or assignment calls for one who is worthy and has the capacity of carrying it out to do so. It isn’t possible for those who aren’t qualified and don’t have the needed potential to get the job done correctly. We also know that Allah’s prophets would receive His messages to them and the people, through revelation; messages that were entirely light and of spirituality, messages containing faith and piety. Clearly, one who intends to receive such a message from the metaphysical world above and the All-Pure and glorious Allah (swt) must be pure of all badness and ugliness and pious enough to be able to do so and receive a message of light and purity.
The reasons for the infallibility of the prophets through the Quran and tradition:
There are some verses in the Quran that express the infallibility of the prophets:
a) The verses that call the prophets “the purified ones”. This term refers to those whom even Shaytan has no hope in misleading. In this verse: “Ibliss said:" By Your honour! I will mislead all of them, except Your purified servants among them.”, Shaytan has excluded those who are of the purified ones from those that he intends to misguide. Mukhlas, or “the purified one”, is the same as infallible, and the prophets are surely part of them. Allah (swt) says: “And mention Musa in the book; surely he was one purified, and he was a messenger, a prophet”, and “He (Prophet Yusuf) was indeed one of purified servants.”
b) The Quran demands the absolute and unconditional obedience of the prophets without any questions asked, and that necessitates their infallibility (for if they were to make mistakes or sin, Allah (swt) wouldn’t demand us to unconditionally obey them).
c) The Quran says that divine degrees are exclusively for those who have no record of injustice, while every sin is at the least injustice to one’s self, and in Quranic terms, sinners are considered unjust. Therefore, one who is a sinner can't be expected to reach any high levels, let alone prophethood.
2- The Imams
The infallibility of the imams doesn’t differ from that of the prophets either; they too are infallible from birth, the only difference is that the imams don’t receive revelation from Allah (swt) and their responsibility is to explain divine teachings and Allah’s revelation for the people.
The rational reasoning for the imams being infallible is the same as that for the prophets.
As for the narrative reasoning on this subject:
“The verse of purification”: “Allah only wants to repel all impurity from you, O People of the Household, and purify you a thorough purification.” In this part of the verse (verse 33 of surah Ahzab) two pronouns have been used which are both plural male ones, having nothing to do with the Prophet’s wives (as some claim) and are pointing to the Prophet’s true household.
The word إنما which means “only”, is used to show that something exclusively belongs to something else and here indicates that what this verse is speaking of solely belongs to the household of the Prophet (pbuh).
The sentence of (یرید...) that is mentioning what Allah (swt) wants, is in reality pointing to what He has done or is going to do, not what He wishes should take place (there are two types of “wanting” in Allah (swt), one is for Him to order something which shows that He wants it, while the other is for Him to desire something and it happens instantly and in other words, says “Be” and it “is”). Here, wanting their purification means that Allah (swt) has purified them, and it can't mean that He has demanded them to be pure, because that isn’t a decree to the Prophet’s household only, that is something that Allah (swt) has wanted from all of mankind.
As for the word رجس, it literally means “an impure” thing, and the reason behind why some scholars define it as “sin”, “shirk”, “false belief” or “selfishness and jealousy” is because all are instances of impurity.
Considering the ال that has come before this word, showing the generality of the word it precedes, this verse is saying that Allah (swt) has removed all impurity from the household of the Prophet (pbuh).
The sentence وَ یُطَهِّرَکُمْ تَطْهیراً, considering that the word تطهیر thatیُطَهِّرَکُمْ is derived from, means purification and is is an emphasis on their purification of all impurities.
Conclusion: Allah (swt) has purified the household of the Prophet (pbuh) from all impurities using different types of emphasis.
Of course there are many other traditions that disclose that the Prophet’s (pbuh) household are only fourteen.
3- The angels
4- Those other than the ones mentioned above (because infallibility has nothing to do with one being prophet or imam, because it is possible for an individual to be infallible and yet not be a prophet or imam) like Lady Fatimah (as) or Lady Maryam (as)
Some verses of the Quran tell us of Lady Maryam’s purity: “O Mary, Allah has chosen you and purified you, and He has chosen you above the world's women.” Commentators believe that this verse is expressing her infallibility. As was said above, the “verse of purification” is expressing the infallibility of the household of the Prophet (pbuh), in which one of them is Lady Fatimah (as).
5- A number of pure people who can reach some levels of infallibility
What is meant here is the level of infallibility that subdues one from committing sin, while not having anything to do with that person mistaking or forgetting sometimes or not. That is because the origin of infallibility is the person’s will. Of course, absolute infallibility solely belongs to a certain group of individuals that were mentioned above, but some of the righteous believers, as a result of self-restraint and self-control and true obedience to Allah (swt), can be infallible and protected from sin in their actions, and this is something that one has to choose. In reality, this is an inner virtue in which causes its possessor to perform one’s wajib acts, stay away from haram, and not insist on committing the “small” sins. This inner virtue is sometimes referred to as the inner virtue of being “just”.
 Aqaidul-Imamiyyah, pg. 187.
 See: Misbah Yazdi, Ma’arefe Quran, vol. 4 and 5, pg.147-212; Amoozesh Aqa’id, vol.2, pg. 232-260, lessons 24-26.
 With help from Question 178.
 Sad:82. قال فبعزتک لاغوینهم اجمعین الا عبادک منهم الـمـخلصین
 Maryam:51. واذکر فى الـکـتاب موسى انه کان مخلصا و کان رسولا نبیا
 Yusuf:24. کذلک لنصرف عنه السوء والفحشاء انه من عبادنا المخلصین
 Baqarah:124. و ما ارسلنا من رسول الالیطاع باذن اللّه
 Baqarah:124. لا یـنـال عـهـدى الـظـالـمـیـن
 Muhammad Taqi Misbah Yazdi, RahnamaShenasi, pg. 673.
 Manshoore Aqa’ede Emamiyyeh, pg. 110-111; Hasan Yousofian, Porsemane Esmat, pg.43.
 انَّما یُریدُ اللّهُ لِیُذْهِبَ عَنْکُمُ الرِّجْسَ اَهْلَ الْبَیْتِ وَ یُطَهِرَّکُمْ تَطهیرا
 Making no difference between that impurity is considered impure by the human nature, by the mind or by religion or by all of them.
 See: Index: The instances of Ahlul-Bayt (website: ).
 Biharul-Anwar, vol. 56, pg. 321. إِنَّ مَلَائِکَةَ اللَّهِ مَعْصُومُونَ مَحْفُوظُونَ مِنَ الْکُفْرِ وَ الْقَبَائِحِ بِأَلْطَافِ اللَّهِ قَالَ اللَّهُ عَزَّ وَ جَلَّ فِیهِمْ لا یَعْصُونَ اللَّهَ ما أَمَرَهُمْ وَ یَفْعَلُونَ ما یُؤْمَرُونَ
 Biharul-Anwar, vol.56, pg. 322, ثُمَّ قَالَ ع لَوْ کَانَ کَمَا یَقُولُونَ کَانَ اللَّهُ قَدْ جَعَلَ هَؤُلَاءِ الْمَلَائِکَةَ خُلَفَاءَ عَلَى الْأَرْضِ وَ کَانُوا کَالْأَنْبِیَاءِ فِی الدُّنْیَا أَوْ کَالْأَئِمَّةِ فَیَکُونُ مِنَ الْأَنْبِیَاءِ وَ الْأَئِمَّةِ
 Al-Imran:42 یا مَرْیَمُ اًِّنَّ اللَّهَ اصْطَفاکِ وَ طَهَّرَکِ وَ اصْطَفاکِ عَلی نِسأِ الْعالَمِینَ
 Al-Mizan, vol. 32, pg. 217.
 See: Allamah Tabatabai, Velayat Nameh, translation of Homayun Hemmati, pg. 48-55.
 Manshoore Aqa’ede Emamiyyeh, pg. 116.