Allāh said in the Qurʾān:
وَإِذِ اِ۪بْتَل۪يٰٓ إِبْرَٰهِـيمَ رَبُّهُۥ بِكَلِمَٰتٖ فَأَتَمَّهُنَّۖ قَالَ إِنِّے جَاعِلُكَ لِلنَّاسِ إِمَاماًۖ قَالَ وَمِن ذُرِّيَّتِےۖ قَالَ لَا يَنَالُ عَهْدِيَ اَ۬لظَّٰلِمِينَۖ (البقرة 124) 
“When his Lord put ʾIbrāhīm to a test with certain Words, and he fulfilled them, He said, ‘I am going to make you an ʾimām for the people.’ He said, ‘And from among my progeny?’ He replied, ‘My promise does not extend to the unjust.’” (2:124)

The Twelvers argued that, since Allāh negated that His promise, which they said is ʾImāmah here, would be given to any ẓālim (unjust person), this means whoever is an ʾimām cannot be someone who sins can come from. This is because all sins are considered ẓulm (injustice), either towards a person’s self or towards another, which means any sinner can be referred to as a ẓālim. This indicates, according to them, that Allāh negated that any fallible person would ever be included in His promise of ʾImāmah. 

It is first useful to mention the different interpretations among the scholars of the Salaf for the “promise” being referred to:¹

1. It is prophethood. This is the view of as-Suddī, and he narrates it from his teachers including Ibn ʿAbbās رضي الله عنهما. 
2. It is ʾImāmah [in terms of rule]. This is the view of Mujāhid and Saʿīd ibn Jubayr, and ʾAbū Ṣāliḥ also narrates it from Ibn ʿAbbās رضي الله عنهما. 
3. It is protection from hellfire. This is the view of Qatādah and ʾAbū ʿUbaydah. 
4. It is Allāh’s mercy. This is the view of ʿAṭāʾ and ʿIkrimah. 
5. It is Allāh’s religion. This is the view of aḍ-Ḍaḥḥāk, ar-Rabīʿ, and ʾAbul-ʿĀlīyah. 
6. It is obedience, and that the word “عهد” here means command, and not promise, meaning that there is no command upon anyone to obey a ẓālim. This is the narration of aḍ-Ḍaḥḥāk from Ibn ʿAbbās رضي الله عنهما. 
7. It refers to reward from Allāh. This is also narrated from Qatādah, and is similar to 3.  

The response to the argument of the Twelvers based on this ʾāyah is as follows:

1.

The promise being referred to in the ʾāyah is not clearly determined and has many possibilities with regards to meaning. It must first be noted that linguistically, the term ʾimām simply means a leader who is followed as an example, and the form of leadership being referred to in the ʾāyah must also be determined. The most reasonable meaning for the leadership here is that of prophethood and being a messenger, and this should also be the meaning of the promise. The leadership of a prophet and messenger is the most complete and perfect form of leadership that can be achieved for a person, and is the highest form of the blessing that can be given here. The ʾāyah states that ʾIbrāhīm عليه السلام was made a leader for the people in a general sense, and also does not restrict his leadership to a specific set of matters. If he were being made a leader without being made a prophet, he would be under the leadership and the legislation of the previous prophet, which would go against the apparent indication of the ʾāyah, and his authority would not be universal. The only form of leadership for which universal following among all people is applicable is the leadership of a messenger. The ʾāyah is thus indicating that ʾIbrāhīm عليه السلام was granted a status of leadership through prophethood and being a messenger such that all of mankind was made to follow his guidance. 

2.

The term ẓālim is most common in the religious texts to refer to kuffār, and there is agreement that kuffār cannot be given ʾImāmah. { وَالْكَٰفِرُونَ هُمُ اُ۬لظَّٰلِمُونَۖ } “And it is the disbelievers who are unjust.” (2:254) The basic linguistic meaning of the term ẓulm is putting something outside of its proper place,² and can include things that are not sins, like in the ʾāyāt of the Qurʾān where this term is used for mistakes of prophets, who all Shīʿī scholars and a large portion of Sunnī scholars agree are free from all (even minor) sins, and all Muslims agree are infallible. Since even the Shīʿī view specifies the term past its base linguistic meaning, there is no objective reason to reject specifying the term to refer to kufr in this context, and this interpretation is supported by the fact that kufr is the most absolute and complete form of ẓulm.  

3.

Even if ẓulm here does not refer to kufr, the opposite of ẓulm is ʿadālah (justice and being free of major sins) rather than infallibility. A person who is fallible but does not sin, or sins and then repents, cannot be referred to as a ẓālim, neither in the language nor in religious terminology. If a person who repented and left sin was referred to as a ẓālim, then by analogy a person who becomes muslim and leaves kufr would still be a kāfir, and this is known to every Muslim by necessity to be false. Such reasoning would even validate calling an old man a child, a dead person living, or an asleep person awake! 

With regards to this ʾāyah, some Twelver scholars have realized the weakness of their argument. ʾĀṣif Muħsinī said:³ “The extent of what is certain about it is the establishment of ʿadālah for the ʾimām, and not infallibility.” 

1.  تفسير القرطبي , تفسير الماوردي , تفسير الطبري , زاد المسير لابن الجوزي , تفسير ابن عطية , البحر المحيط لأبي حيان , الدر المنثور للسيوطي  
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2.  http://arabiclexicon.hawramani.com/ظلم/ 
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3.  مشرعة بحار الأنوار لآصف محسني – ج2 ص453 
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