And when He overcame you with sleep, making you feel secure, and sent down water from heaven to purify you and remove the taint of Satan from you , and to fortify your hearts and make your feet firm. (Surat al-Anfal, 8:11)
Human beings make mistakes throughout their lives. No doubt, believers make every effort to avoid such mistakes and strive not to commit them by living according to Allah's religion. This fact is related in the Qur'an, as follows:
If Allah were to take mankind to task for what they have earned, He would not leave a single creature crawling on it; but He defers them until a specified time. Then, when their time comes, Allah sees His servants! (Surah Fatir, 35:45)
In compliance with this divine command, sincere believers are not expected to be infallible or sinless; rather, they are expected to seek Allah's pleasure, abide by the limits that He has established for them, turn to Allah in repentance, and take refuge in His mercy. As the Qur'an informs, Allah will purify them, for Allah “desires to remove all impurity from His servants.” This is related in the following verse:
... Allah desires to remove all impurity from you, O People of the House, and to purify you completely. (Surat al-Ahzab, 33:33)
Although a person may have committed great sins, rebelled against Allah, and lived by principles that are utterly contrary to Allah and His religion, Allah is so compassionate that He may purify those of His servants who sincerely repent. Furthermore, Allah informs us that He may even purify and forgive unbelievers and hypocrites who have fought Him and His religion if they ever turn to Him with a sincere heart. One verse reads:
The hypocrites are in the lowest level of the Fire. You will not find anyone to help them. Except those who repent, put things right, hold fast to Allah, and dedicate their religion to Allah alone; they are with the believers. Allah will give the believers an immense reward. (Surat an-Nisa', 4:145-46)
These words are manifestations of Allah's infinite mercy toward His servants. Clearly, only He can purify His servants from idolatry, evil, and spiritual uncleanliness. Allah is the most compassionate of the Compassionate.