(8) لِيَسْأَلَ الصَّادِقِينَ عَن صِدْقِهِمْ وَأَعَدَّ لِلْكَافِرِينَ عَذَاباً أَلِيمَاً
8. "That He might question the truthful of their truthfulness, and He has prepared for the unbelievers a painful chastisement."
Allah takes a covenant from the Prophets and He has an aim in it. Its aim is to separate the truthful ones from the disbelievers. Thus, being far from truthfulness is being nigh to infidelity.
This verse states the aim of the mission of the prophets and the firm covenant that has been taken from them as follows:
and He has prepared for the unbelievers a painful
The Islamic commentators have delivered a great deal of interpretations upon the purpose of the Qur'ānic term /sādiqīn/ (the truthful) here that who they are, and that what question is this question? The matter, which seems harmonious with the verses of this Sura as well as other suras of the Qur'ān, is that the purpose is to define the believers who have proved the truthfulness of their own claim in action. In other words, they have been successful in the trial of Allah. The evidence for this statement is as follows:
1- The term 'Sādiqīn' (the truthful) here is mentioned versus 'Kāfirīn' (disbelievers) and this meaning is understood well from the context in opposition of this concept.
2- We recite in verse No. 23 of this Sura (Al-'Ahzāb): "Among the believers are men who were true to their covenant
with Allah; …" Immediately after that statement, in verse 34, the Holy Qur'ān explains the aim of it by saying: "That Allah may recompense the truthful ones for their truthfulness, and chastise the hypocrites if He please, or turn unto them (mercifully), …"
3- Sura Al-Hujurāt, No. 49, verse 15 and Sura Al-Hashr, No. 59, verse 8 introduce the truthful ones clearly. The Qur'ān says: "The believers are only those who believe in Allah and His messenger then they doubt not and struggle hard with their wealth and their lives in the way of Allah; and these are the truthful ones." And, concerning the booties that Muslims gain without fighting, it says: "(It is) for the poor who fled, those who were driven from their homes and their possessions, seeking grace of Allah and (His) pleasure, and assisting Allah and His messenger, these it is that are the truthful."
Thus, it is clear that the purpose of the term /sādiqīn/ is those who have proved their truthfulness in the fields of supporting the religion of Allah, Holy struggle and perseverance against the difficulties and giving their wealth and lives in the path of Allah.
Now, what is the main purpose of the question from the truthfulness of the truthful ones? Regarding to what was said in the above, it is clear that the purpose is that whether they prove the sincerity of intention and the truthfulness of their claim in their deeds or not. Among their deeds there are: payment of the alms-tax, Holy struggle, and showing patience and perseverance against the difficulties and the hardships of the battle-field in particular.
Where will this question be asked? The apparent of the holy verse shows that it will be asked in Hereafter and in the
 Sura Al-Hujurāt, No. 49, verse 15  Sura Al-Hashr, No. 59, verse 8
just court of Allah. There are also numerous verses in the Qur'ān that inform of the performance of this question wholly in Hereafter. But there is also this probability that the question may have a practical aspect and it performs in this world, because by the mission of the Divine prophets all those who claim faith will be questioned and their action is an answer to this question that whether they are truthful in their claim or not.
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