The Holy Book tells us about the Words of Allah, the Great and Almighty, concerning them:
" Allah only desires to keep away uncleanness from you, O members of the House! and to purify you a (thorough purification)." (XXXIII:33)
It is a virtue for them. No one of all people has this virtue. There is no dignity more precious than keeping away uncleanness from them and purifying them from all defects, that is the purifying which the Most kind (Allah) likes for them with His care, and it is not limited to a certain uncleanness or from a certain thing. So, it implies general purifying from every defect and sin. From this holy verse, we conclude the infallibility of the members of the Prophetic House (Ahlul-Bayt), because every sin is uncleanness and committing sins does not agree with their keeping away from them and their purification from them, so, they are, according to this verse purified from uncleanness and sins.
Is infallibility a thing behind this? Yes, all importance is in the concerned persons with this virtue which no one of the community had, except them. Does this virtue concern those who were in the House when the above- mentioned Holy Verse came down or does it concern every one who belonged to the most purified Prophet because of a certain affinity or ancestry? If the second question is accepted, then the reality opposes it, because some of the Prophet's wives disobeyed him, and no uncleanness is greater than this disobedience. So, certainly his wives are not concerned with this virtue. Also some of those who belonged to the Prophet because of ancestry committed grave sins and crimes, and it is not right that Allah, the Almighty, wants to do a thing by the creation will, (1) but the thing does not happen. As it is impossible that Allah wants to create a thing and the thing does not happen, so all the Prophet's wives and all the Hashimites are not included in the foregoing verse, because the women and the Hashimites might commit what conflicts with purification, besides no one has said that all his wives and all the Hashimites were infallible. And if the intended meaning in the verse was the legal will, then no sense would the will of purification concerning the members of the House (Ahlul-Bayt) have because Allah, the Exalted, wants it from all people. As the verse concerns the outstanding merit of Ahlul-Bayt, it indicates that the creation will is confined to them. Then the legislation will is related to the acts of creatures, while the verse concerns the act of Allah.
Therefore, if the verse was related to the legislation will, He (Allah) would say:" Go and purify yourself." So, there is no doubt that the concerned persons in the verse are those who have been mentioned in the first question, namely the intended meaning of the verse refers to certain people, and they were those who were in the House of our master Mohammed, may Allah bless him and his family, where he covered them and himself with his cloak (kisa'). So, the verse was sent down to them and concerning them. They were `Ali, Fatima, and their sons, peace be on them. Moreover the correct traditions of the two parties (Shi'a and Sunna) affirm that. And if there was no religious text that denotes that the verse concerns them (Ahlul-Bayt), then their behavior would be the greatest evidence for the relation between them and the verse, because their actions and sayings force us to admit their honesty. This plain truth has been well known for the clear- headed persons since the revelation of this clear verse till today.
They certainly know that the members of the House (Ahlul-Bayt) were the people of the cloak (kisa'), who had countless noble qualities of which were purification from defects and taking away uncleanness and sins. Yes, maybe, some of the Hashimites made use of the general sense of the word (Ahlul-Bayt) to achieve their aims and to reach the throne, so all the Hashimites used this verse to convince people to follow them. Also the word 'Shi'ism' (tashayy`) maybe used to refer to 'Ali and (the members of the) House (Ahlul-Bayt) in the general sense, not the members of the cloak (kisa') and the Imams from the sons of al- Husayn, peace be on them, but the persons whom the floods of the rabble do not deviate them from the truth. People knew that Shi'ism (tashayy`) was limited to those certain Imams when silence spread over them after the first one- third of the Abbasid dynasty when the revolts of the Alawids were over. Our evidence is that the Abbasid were able to destroy the Marwani dynasty and to take the reins of government by using the words (Ahlul-Bayt).
The Abbasid used these words to make the hearts of Shl'a incline to them and to make them a bridge to build an entity for their authority and to destroy the entity of the Omayyad dynasty that resisted the members of the House (Aahlul-Bayt) and their followers throughout its' days and colored the surface of the ground with their shed blood. Thus the Abbasids achieved their ambition through the members of the and their the followers House (Ahlul-Bayt). They used them both as a bridge to achieve their goals. Generally speaking they madeuseofall parties: the Abbasid, the Talibis, the Alawids, the Ja'farids, the 'Aqilis, the Hassanis, and Husaynis.
Then every Hashimite used the same manner to summon people to support his rising. For example, Abd Allah b. Mu'awiya b. Abd Allah ibn Ja'far revolted against Abu Muslim in Kufa then in Persia because they had the followers of the members of the House (Ahlul-Bayt), but Abu Muslim was able to kill Abd Allah when his followers abandoned him. Besides Zaid and his son, and the two brothers Mohammed and Ibrahim rose to take the authority from the enemies of the members of the House (Ahlul-Bayt). After that, people realized that the Abbasids did not belong to the members of the House (Ahlul-Bayt) because they pulled out the sword of aggression against the members of the House (Ahlul-Bayt), who were the kinship of the Prophet, may Allah bless him and his family. Moreover, people knew that the Abbasids wanted to achieve their aims when they summoned them to overthrow the Omayyad dynasty under the pretext of avenging the blood of those who were killed in (the Battle of) al-Taf and the crucifixion of Kanasah and Jawzjan and the like.
That was because the Abbasids had built their authority on the skulls of those who loved the members of the House (Ahlul-Bayt). In other words, they showed the spite of their chests and their evil inner most thoughts to ward Ahlul-Bayt and their followers. Besides, Mohammed and Ibrahim disappeared when al- Safah took the reins of government because they were aware of his intentions toward the close relatives of the Prophet. (Muhammad Al-Husayn Al-Muzaffar, Imam al-Sadiq, p.21-23)
source : http://www.maaref-foundation.com