Infallibility

One of the most distinguishing features of the prophets is their infallibility.
They were infallible throughout the entirety of their lives and immune from all types of major and minor sins (whether intentional or unintentional) and also from errors (like forgetfulness, inadvertence, etc.). They are also infallible in a threefold manner with regard to revelation:

A: In receiving the revelation
B: In preserving revelation
C: In conveying the message to the people

The infallibility of the prophets does not merely equate to the absence of committing sins, because it is possible for an ordinary person not to commit sins, particularly if he/she has a short life. Rather, it means that the person should have an innate power, which protects him against committing sins even in the hardest situations. Such a power is the result of a perfect and permanent awareness of the disgracefulness of sin and having a strong will to control his/her own ego. Since such ability is only formed by God's help, its functioning is assigned to Him. However, it is not correct to think that God forcefully protects a person against committing sin and as a result deprives him/ her of freedom. The infallibility of those who perform their heavenly responsibilities like the prophets (as), and Imams (as) is related to God in one more way, that is: God has guaranteed their immunity. 

Intellectual Reasons for the Infallibility of Prophets (as)

There are many arguments for the infallibility of the prophets. Among them two intellectual reasons are more popular. The first intellectual reason is that the main objective for their mission is to guide man towards the truth and teach him how to perform the duties God has assigned to him. Moreover, the prophets are in fact the representatives of God among humankind and must guide them towards the right path. However, if such representatives and missionaries do not obey the divine instructions themselves and their behavior contradicts their mission, people will find the contrast between their words and behavior contradictory and, therefore, the objective of their mission will not be thoroughly fulfilled. Thus, God's wisdom and kindness necessitates that the prophets be pure and innocent, and for them to refrain from committing any undeserving action.