“Sin” is the lack of obeying and fulfilling the orders of Allah (SWT). There are two kinds: There exists unintentional sin and disobedience that is due to ignorance, lack of ability to obey due to weakness of the brain, self, or body, like the disobedience of Prophet Adam (AS) before his prophethood, and also like many of the people.
There also exists intentional sin and disobedience that occurs with the presence of knowledge, and perfect ability to obey, like the sin and disobedience of Iblees (Shaytan) and his followers from the human beings and Jinn.
Allah (SWT) may forgive the first type of sin and disobedience with His compassion and generosity if the sinner regretted and repented from his sin with a sincere intention. As for the second type of sin, its consequences are the expulsion of the sinner from the mercy and compassion of Allah (SWT), and the eternal punishment and damnation of that person.
The Prophets, Messengers, Awsiya', and Imams are purified and protected from committing both kinds of sins and disobediences. When Adam (AS) “disobeyed” Allah (SWT), this happened when he was still in Paradise before he descended to Earth and before he became a Prophet. When he was appointed as Prophet, he became purified and protected, and so, we never saw from him any kind of disobedience after that.
Here, we need to clarify and stress an important thing, which many people fail to recognize and understand. Disobedience is the act of not obeying and not executing the command of Allah (SWT). And Taharah and ‘Ismah of the Prophets is from this act of disobedience to the orders of Allah (SWT) that are directed to them in any matter.
The meaning of disobedience here is not what the minds are used to when they think of a sin because some behaviors are generally considered to be a sin. The most important thing that one has to keep in mind is that the meaning of disobedience is specifically not obeying and carrying out the orders of Allah (SWT). It is not what comes to your mind from specific actions which we are used to calling a sin.
It is important to differentiate between these two scenarios even if they coincide in most cases. For example, lying, killing, stealing, and wrongly accusing others are all actions which we consider to be a sin, and they indeed are. However, they are not considered to be a sin due to the bad nature of the action in itself.
Rather, they are considered to be sins because Allah (SWT) forbid and prohibited them. So, killing in itself is not a sin, but Allah's forbiddance is what makes it a sin if committed. This is a very important concept that we need to understand and recognize to avoid confusion and so that we can later understand the meaning of Taharah and ‘Ismah, like Allah (SWT) willed.
So, if a person does not lie, in adherence or accordance to good behavior and commendable traits, this would not be considered “obedience to Allah (SWT)”, and he would not be characterized as “obedient” to Allah's order in this case. He is certainly not equal to one who does not lie because of his adherence to Allah's prohibition to lying.
In general, Allah (SWT) orders us to be truthful and honest. However, in certain and specific situations, He permits us to lie, and may even order us to do so. For example, lying on the enemies of Allah (SWT) and the enemies of the Muslims at the time of war is permissible, in order to avoid revealing the secrets of the Muslim army to the enemy.
You are allowed to mislead them, and the appearance of this action is lying. However, its reality is obedience to Allah (SWT) and what's important here is the obedience of Allah (SWT). For instance, a person may lie in order to bring two angry and conflicting believers together, and this lie is justified and encouraged, as Allah (SWT) says in the Qur’an,
وَأَصْلِحُوا ذَاتَ بَيْنِكُمْ

"Wa aslehoo dhata baynekom."
“..And adjust matters of difference among you…” [8:1].
So, the action in its outward appearance is a lie and a sin, but it is permissible since Allah (SWT) allowed for that in such an exceptional situation. Furthermore, in its outward appearance, Taqiyya (dissimulation) may be interpreted as lying, but Allah (SWT) has ordered us to observe it in special cases in order to protect ourselves, our money, property, and our family.
Allah (SWT) and the Holy Prophet (SA) have excused ‘Ammar ibn Yasser (RA), when he was forced by the nonbelievers to curse the Prophet (SA). ‘Ammar thought that he was doomed by committing that act.
So, Allah (SWT) revealed the following verse,
إِلاَّ مَنْ أُكْرِهَ وَقَلْبُهُ مُطْمَئِنٌّ بِالْإِيمَانِ

"Ilaa wa qalboho motma-enon bil imaan."
“…Except him who is forced thereto and whose heart is at rest with fait…”, [16:106].
So in these special situations, the order of Allah (SWT) is separated from the typical connotations of the act. In such cases, we are to abide by the orders of Allah (SWT) even if we commit that sin-like act. That is because the only important thing that matters is the obedience of Allah's orders, not what our minds and traditions accept or not. This way, we will be able to interpret many of the events/incidents that occurred without conflicting with the meaning of ‘Ismah.
For example, Allah (SWT) ordered Prophet Ibraheem (AS) to deceive his people by claiming to be sic, in order for him to have an opportunity to demolish their idols after their departure. Here, the appearance of his act seems to be a lie and therefore a sin. However, its reality is in abidance and obedience to the order of Allah (SWT) to him, which is what matters most.
Prophet Yusuf (AS) falsely accused his brothers of stealing the drinking cup of the king, and this, in the eyes of any person, is considered to be a sin. However, Allah (SWT) ordered him to do so and He (SWT) said in the Qur’an,
كَذَٰلِكَ كِدْنَا لِيُوسُفَ

"Kadhaleka kedna li Yusuf"
“…Thus We did plan for Yusuf…” [12:76].
Yusuf (AS) is purified and protected from disobeying Allah (SWT), even if his act seems wrong in front of the common eye. Once again, what matters most is the absolute obedience to the orders of Allah (SWT).
Hence, there is a big difference between the orders of Allah (SWT), and the typical connotations of actions even though they may coincide in most cases. We must realize that the orders of Allah (SWT) is the most important, not the act in itself. And the examples in this regards are many.
Eating is permitted, however during the days in the month of Ramadan, it is forbidden. Killing a human life is forbidden, however, Allah (SWT) orders us to use the Law of Equality in certain cases and He permitted us to kill His enemies.
Sexual relations with the opposite gender is forbidden except with marriage or with the slaves whom your right hand possesses…and so on and so forth. We see actions that are permitted in some cases and forbidden in others. And in certain times, they're allowed while in others, they're prohibited.
The line to be drawn here and the determiner, is the obedience of Allah's commands because that is our ultimate goal and the desired aim. This is what Allah (SWT) always tests us with even if His order contradicts the desires and traditions of the people. In fact, these are from among the important choices that a believer has to make.
For example, Ameer Al-Mu’mineen (AS) did not stand to take arms in seeking his divine right of Caliphate from those who confiscated it from him. This was in accordance to the orders of Allah (SWT) and Rasulullah (SA), even if the people and history blame him for that, for he is purified and protected from ever disobeying Allah (SWT).
Imam Al-Hasan (AS) agrees to make a peace treaty with Mu'awiya in accordance to the orders of Allah (SWT) and Rasulullah (SA), even if the people and history blame him, for he does not care because he is purified and protected from ever disobeying Allah (SWT).
Imam Al-Husain (AS) rises up to face Yazeed (LA) and he takes his family with him (while he knows that his sons, companions, and himself will be killed), in accordance to the order of Allah (SWT). He also knows without a doubt that his women will fall as captives under the hand of the enemy. However, he is purified and protected from disobeying Allah's orders, and he takes that decision no matter how much the people or history blame or criticize him.
Imam Al-Sajjad (AS) stays at home and engages in worship, while Banu Ummaya widely spreads corruption in the Islamic nation. He doesn't leave his house in accordance to the orders of Allah (SWT) even if the people and history blame him. That is because he is purified and protected from disobeying Allah (SWT) not even for a blink of an eye, even if obeying Allah's order will seem strange, unusual, or contradicting to the mind and traditions.
Al-Khidr (AS) slaughtered a young boy and scuttled the ship. In its outward appearance, these actions are considered to be sin such that Prophet Musa (AS) could not bear it. However, Al-Khidr carries out the orders of Allah (SWT) whether or not the people like it, and whether or not it coincides with their logic, habits, and opinions. As the verse in the Qur’an says,
وَمَا فَعَلْتُهُ عَنْ أَمْرِي

"Wa ma fa'altoho 'an amry..."
“….And I did them not of my own accord…” [18:82].
These are all lessons for us so that we can realize that's what is important is obeying the commands of Allah (SWT), not the appearance or connotations of the actions!
Taloot goes out with a small and limited number of a small army to face a huge army of tyrants. Regardless of that difference in number, the orders of Allah (SWT) come to him (which he obeys) that leads to the decrease in the quantity of the army. Here, the appearance of the situation contradicts the human logic in both military and political smartness.
However, Taloot simply carries out the orders of Allah (SWT), even if it leads to the decrease of the number of his army or their defeat, because that is what matters most and that is the reason for the creation and its presence. As Allah (SWT) says in the Qur’an,
وَمَا خَلَقْتُ الْجِنَّ وَالْإِنْسَ إِلَّا لِيَعْبُدُونِ

"Wa ma khalaqto al ens wal jenn illa liya'bodoon"
“And I created not the Jinn and mankind except that they should worship Me” [51:56].
In sum, the main and true essence of worshipping is the absolute and complete obedience and submission to God’s commands regardless of what you like and what you don't like, or what the people like or don't like. What matters is the obedience of Allah's orders and commands wherever you are and in whatever condition you are in.
The Taharah and ‘Ismah of the Prophets, Messengers, Awsiya', and Imams is in essence from the disobediences of Allah (SWT), regardless of the appearance or connotation of the actions and words, and of the opinions and desires of the people. Thus, the commands of Allah (SWT) are on top of everything, before everything, and apply to everything.
After all, did the angels become angels except by their absolute obedience to Allah (SWT)? And did the creations and kingdoms get created, except by the order of Allah (SWT)?

Source: shiastudies