Why should the prophets be free of sin and error (infallible)? Please explain.

Question:
Why should the prophets be free of sin and error (infallible)? Please explain.

Answer:
Without doubt, more important than anything else, a prophet must attract the trust of the general public in such a way that his words contain no possibility of being lies or erroneous, otherwise, his position of leadership will be a shaky one.

If they are not immaculate, using the excuse that the prophets have erred, people who seek the truth from what they say will begin to doubt their invitation. It will not be accepted, or, at least, their words will not be accepted with all of their hearts.This reason which can be called ‘trustworthiness’ is one of the most important reasons for their being immaculate.

In other words, how is it possible that God give His Commands for His people to follow a person who is not truthful for if this person were to err or sin, people would not follow him. If they do, they have erred and if they do not, they have weakened his position of leadership, in particular, since the position of the leadership of the prophets completely differs with the leadership of others for people receive their entire program of life from the prophets.
Because of this, we see that the great commentators speak about the verse,﴿يا ايها الذين آمنوا اطيعوا ا لرسول و ا ولي ا لامر منکم﴾“Obey God and obey the Prophet and those charged with authority among you.” (4:59)

saying that the command for Absolute obedience is because the Prophet is immaculate as well as ‘being charged with authority’. The pure leaders like the Holy Prophet are referred to as ‘being charged with authority’. If not, God would never give the command to unconditional obedience to them.
Another way of proving the immaculateness of the Holy Prophet in relation to any sin is that any factor of sin is condemned to defeat within the very being of the Holy Prophet.

The explanation of this is: when we turn to ourselves, we see that we, too, are immaculate in relation to some sins or evil or unacceptable deeds.

Note the following examples:
Can you find an intelligent person who thinks about eating fire or trash or filth?
Can you find an intelligent person who will walk naked through the streets and bazaars?
Clearly not! If we saw such deeds from someone, we will be assured of the fact that he is no longer normal and has become insane because an intelligent person would never do these things.

When we analyze such behavior, we see that the ugliness of such deeds is so clear that an intelligent person would never even consider them.
It is here that we can imagine what this short phrase means and say that every intelligent and healthy person is ‘free of’ unacceptable deeds.
From this stage, we take a step further. We see some people who are free from unacceptable deeds.

For instance, an aware physician and expert who knows the various kinds of microbes well is never prepared to drink the polluted water of the dirty clothes of a person who has one of the most dangerous contagious diseases whereas an illiterate person , perhaps, would be indifferent to such a thing.
With another simple example, we reach the point that however much the level of a person rises in the area of awareness, they are less likely to do evil or ugly deeds.

Taking into consideration that if a person’s faith and awareness were to rise and have so much faith in God and His court of justice, so that everything that he sees is present before his eyes, such a person will be free of all sin and every ugly deed in relation to him, like walking naked through the streets, will be in our eyes only.

For such a person, the property of something forbid den is just like the flames of fire, and just like we do not put fire in our mouths, he does not put something which is forbidden into his mouth.

We can then conclude that the prophets, because of the extraordinary knowledge, awareness and faith which they have, tame the motives of sin and the most exciting factors causing sin will not prevail upon his intellect and faith. This is why we say that the prophets are immaculate; they are insured against sin.

Refrence:

Ref: Fifty Lessons of Principles of Belief for Youths; Ayt. Makarem Shirazi; Translated by: Bahador Shirazian; Published by: Ahlul Bayt World Assembly