Why do Shiites consider the Imams ‘Ma`sum,’ infallible?

Answer:
There are several reasons proving that the Imams of the Shiites, all members of the household of Prophet Muhammad (Ahl al-Bayt), are all infallible (i.e. protected against errors and sins). Only one of the reasons is presented here:

According to Shiite and Sunni scholars, the blessed Prophet, in the last days of his life, said, “I leave behind two invaluable things for you: the Book of Allah and my Ahl al-Bayt. They will never be separated until they meet me at the fountain of Kawthar.”[1]

[1] Al-Hakim's al-Mustadrak, part 3, p. 148; Al-Sawa`iq al-Muhriqah, part 11, chapter one, p. 149. Similar texts are mentioned in Kanz al-`Ummal, part one on al-I`tisam bil-Kitab wal-Sunnah, p. 144; Musnad Ahmad, part five, pp. 182 and 189.

There is no doubt that the Qur’an is secure from any deviation and mistake. How can there possibly be any mistake in the divine revelation sent down by God, carried by Gabriel, and received by the blessed Prophet? It is as clear as daylight that all the three are free from any error.

 Muslims maintain that the Holy Prophet is safe from any errors in receiving, guarding and conveying the revelation. With the book of Allah so truly and firmly free from any error, it is obvious that the household of the Prophet, the Ahl al-Bayt, are also secure from errors and mistakes, for the household have been paired with the Qur’an in the tradition cited above. Thus, since the two have been paired up with the Qur’an, they must be infallible in guiding and leading the people. Put another way, there is no reason why an erring person should be paired up with the Qur’an.

The clearest evidence to the infallibility of the Imam is the words of the holy Prophet, who said, “These will never be separated until they meet me at the Fountain of Kawthar.” Should the household of the Prophet not be protected against errors, or should they ever err, they will get separated and deviate from the error-free Qur’an; this, nonetheless, has strongly been refuted.

Certainly “the household” cited above does not mean all the prophet’s relatives, in-law or ancestral, for surely not all of them were secure against errors. Therefore, the honor is peculiar to some members of his family, the Imam, who have been beacons for the Ummah, and guardians of the prophetic Sunnah and the Shariah (the religion) throughout history.

Also, twenty-six Muslim scholars have written separate books on the recording and handing down of this tradition. For instance, the famous Muslim historian, Abu-Ja`far Tabari wrote two volumes of books on this issue. `Allamah Amini’s book Al-Ghadir, provides further information in this regard.