Infallibility is the inability to be wrong. Its importance and meaning is debated in epistemology and major religions.
Yes, every prophet had a successor who leads his Ummah after the Prophet and all of them are infallible. The Hadeeth لكل نبي وصي is narrated in Shia and Sunni books. Among Sunni scholars who narrated this Hadeeth are Ibn Hanbal in Fadha'l al-Sahabah, Sibt Ibn al-Jawzi in Tathkirat al-Khawaass, Ibn al-Maghaazili in al-Manaaqib, al-Tabaraani in al-Mo'jam al-Kabeer and Ibn Asaakir in Tareekh Dimishq and many others.
The fact of the infallibility of Ameerul Mo'mineen and other 11 Imams was known to the believers during the lives of the Infallible, and nothing emerged later in hindsight.
All the prophets and their divine successors are infallible and sinless as they are the best examples to be followed by human beings. Allah (SWT) ordered all human beings to obey the prophets and their successors. If God forbid, they do any sinful act, it means that Allah (SWT) is ordering us to commit sin following them, which is impossible.
Sura al-Nasr like many other Surah have been mistranslated. It does mean at all to seek forgiveness for the prophet himself but for the Muslims who enter in the Religion of Allah for their past sins.
Every verse in Quran and every Hadeeth on this matter means that the Prophet is seeking forgiveness for the Believers.
Thank you for your question. Each person's reward is in accordance with the tests they face in relation to the gifts and abilities they have been given. As such people who are not born infallible will not be judged in the same way that the Infallibles (as) are. But at the same time, they are also not tested with the responsibilities of the Infallibles (as), nor are they rewarded in the same way. Infallibility is a knowledge that keeps a person away from sin and mistakes. It does not remove the free choice and therefore does not negate the applicability of reward for great actions.
May you always be successful
The Prophet (SAWA) did not pray to Allah to keep Ahlul Bayt (AS) away from Rijs, but he prayed to Allah to keep the Rijs away from Ahlul Bayt (AS).
This is a misunderstanding in the minds of many who do not realize the difference between (A'nhom) عنهم , and (Minhom) منهم or error in the translation.
A'anhom: means to drive the bad away not to be near them.
Minhom : means to drive the bad from them.
Allah (SWT) never said Minhom, but said A'nhom.
انما يريد الله ليذهب عنكم الرجس أهل البيت ويطهركم تطهيرا (33:33)
Ahlul Bayt (AS) are The Infallible Divine leaders who are completely purified by Allah (SWT) who stated in Quran that He wants to to keep the bad away from them because they have nothing from the bad at all.
It is Not correct at all to claim that the Prophet Muhammad (SAWA) was the least imperfect human.
There is not a single evidence about any imperfectness is the life of Prophet (SAWA).
The Prophet Muhammad (SAWA) is the Most Perfect Human and The best of All the Prophets.
The meanings of Quranic verses must be taken from the Prophet himself as Allah (SWT) said in Quran وأنزلنا إليك الذكر لتبين للناس ما نزل إليهم
( We revealed on you the Thikr ( full knowledge of Quran) So that you explain to the people what was revealed for them). Al-Nahl, 44)
Verses which seem to suggest that any Prophet was fallible, are been misunderstood. Verses regarding Prophet Adam (AS) are been explained by the Prophet Muhammad (SAWA) and his Holy Progeny confirming that Adam did not commit any sin whatsoever.
Ahlul Bayt (AS) had many debates in this subject including a very well known debate between Imam Al-Redha (AS) and Al-Ma’moon al-Abbasi, the ruler of that time, in which al-Ma’moon put to the Imam ( AS) the Quranic verses which were according his understanding, showing fallibility of some prophets, and Imam Al-Redha (AS) explained the real meaning which confirms infallibility of all the Prophets.
You may see Tanzeeh al- Anbiyaa’ by al-Shaikh al- Mofeed. تنزيه الأنبياء للشيخ النفيد.
The claim of fallibility of any Prophet goes against Quran where Allah orders people to absolutely obey the prophets. Allah will never order people to obey a sinner.
As a result of political reasons which made many Muslims who blindly follow governments of Saqeefa, away from the teachings of Ahlul Bayt ( AS) which are the real real teachings of the Prophet Muhammad ( SAWA), Muslim sects who went away from Ahlul Bayt, deprived themselves from the most authentic Islamic knowledge and took instead from outsiders. Thousands of Israelites have come into Muslims books, such as the claim that the prophets are fallible. Jewish literature attacks many prophets and even accuses them of committing major sins like adultery and drinking wine. These false ideas have penetrated into Muslims who were away from Ahlul Bayt (AS) and made many of them take such Israelites as part of their religious knowledge.
Many Sunni Shaikhs say that the Prophets are infallible in Tableegh only but fallible in their usual acts. This claim goes against what Allah (SWT) has said in Quran when He ordered all human beings to absolutely obey the Prophets.( We did not send any messenger but to be obeyed by order of Allah)(Al-Nisaa’, Verse 64). How such absolute obedience can be possible if any Prophet is fallible? Does Allah order us to obey a sinner even for a moment? That is impossible.
Some Quranic verses have been misunderstood or deviated from the real meaning which was explained by the Prophet (SAWA) and taken in accordance with non Muslim text supporting the false claim against the prophets.
Ahlul Bayt (AS) narrated the prophetic earnings of Quran which states that all the prophets are infallible.
All Shia and Sunni scholars believe the the Faqeeh has authority in the interest of the religion and Muslims. Shia scholars have discussed in their books the limits of this authority as a matter of scientific research. While all of our scholars believe in the authority of the Faqeeh, they have different opinions about its limits and boundaries . Al Naraqi was a well known Shia Scholar who wrote about this subject, but no scholar said that the authority of Faqeeh is equal to authority of infallible Imams.
Thank you for your question. An infallible is somebody who is protected from committing sin as their knowledge and faith are such that it would be completely irrational (and therefore impossible) for them to commit any kind of sin. Their purification is to such an extent that their commanding soul does not call them to sin as it is completely under the control of their intellect.
An infallible is a sinless person, but being sinless is more general and could also include someone who hasn't committed a sin yet but perhaps doesn't have the knowledge and faith to make them infallible. The two are therefore the same in that they haven't committed sin, but can also be different from the perspective of knowledge and faith. An infallible is sinless and always will be.
May you always be successful.
The name of Ahl al Bayt are not limited to Imam `Ali, Bibi Faatima, Imam al Hasan and Imam al Husayn `Alayhimu Ssalaam because Rasul Allah (Salla~llahu `Alayhi Wa Aalihi wa Sallam) is reported to have prophesied the names of the Imams from his Ahl al-Bayt that would follow. In a Hadith reported, he publicly announced them by name; and then each one of them is reported to have announced his successor.
The Twelve names that Rasul Allah Salla~llahu `alayhi wa Aalihi wa Sallam announced are:
(1) Imam `Ali ibn abi Talib (2) Imam al-Hasan ibn `Ali (3) Imam al-Husayn ibn `Ali (4) Imam `Ali ibn al-Husayn (Zayn al Aabidin) (5) Imam Muhammad ibn `Ali (al-Baqir) (6) Imam Ja`fer ibn Muhammad (al-Saadiq) (7) Imam Musa ibn Ja`fer (al-Kaazim) (8) Imam `Ali ibn Musa (al-Rida) (9) Imam Muhammad ibn `Ali (al-Jawaad) (10) Imam `Ali ibn Muhammad (al-Haadi) (11) Imam Hasan ibn `Ali (Hasan al-Askari) (12) Imam Muhammad ibn Hasan (al-Mahdi)
May Allah's Peace be upon all of them as we await the return of our Twelfth Imam, also as per prophesy from Rasul Allah Salla~llahu `Alyhi wa Aalihi wa sallam.
By virtue of the above Hadith, they too are part of the Ahl al Bayt.
Since they are the Imams appointed, they too are beyond making any sins,and therefore, Ma`sum.
As to your question of why are they sinless or what does the sinlessness mean, I think it is very important to know that their sinlessness does not mean that they are not human beings like ourselves. We are not allowed to exaggerate anything about Imams and it is our fundamental belief that they are human beings like ourselves and are bound by all the Laws and Hukm of Allah Subhanahu wa ta`ala.
They do not commit sins because they are honoured servants of Allah; and Allah Subhanahu wa ta`ala has honoured them and has given them great dignity and authority. Allah has protected them from all kinds of uncleanliness and has throroughly purified them (see Qur'an 33:33).
They have the highest perfection and are endowed with `ilm al-ladunni and chastity. They are endowed with all virtues, bravery, chastity and quality. Further, they are the carriers and transmitters of the correct Sunnah of Rasul~Allah. They are therefore our Imams. They are the guides and they have the authority after Rasul Allah(Salla~llahu `Alayhi wa Aalihi wa Sallam) in the matters in which we require help and guidance; in religious commandments, in Judgment, in legislation, in the commentary of the Qur'an and in the interpretation of the Qur'an.
For further reading in this and other issues that relate to us please see a very concise book "The Faith of Shia Islam" by Muhammad Rida al-Muzaffar. Truly Allah IS the Knowing.