الحمد لله رب العالمين، وصلى الله وسلم على نبينا محمد وعلى آله وأصحابه أجمعين، أما بعد
Imaam Ibn Rajab (rahimahullaah) said:
“From the factors in attaining forgiveness is: Tawheed, and it is the greatest factor, so whoever is bereft of it is bereft of forgiveness, and whoever comes with it; then he has indeed come with the greatest of factors for the attainment of forgiveness.
Allaah, The Most High, said:
إِنَّ اللَّهَ لَا يَغْفِرُ أَن يُشْرَكَ بِهِ وَيَغْفِرُ مَا دُونَ ذَٰلِكَ لِمَن يَشَاءُ
((Indeed, Allaah does not forgive Shirk [association in worship] being made with Him, but He forgives what is other than that for whom He wishes.)) (An-Nisaa: 48)
So whoever comes – along with Tawheed – with the equivalent of the earth; and it is that which fills it or that which comes close to filling it in wrongdoing; then Allaah will meet him with its equivalent in forgiveness. However; this is with the will of Allaah, The Mighty and Majestic, so if He wills; He will forgive him, and if He wills; He will seize him by his sins. Then his outcome will be that he will not remain eternally in the Fire, rather he will exit from it – then enter Paradise.
Some of them have said: the Muwahhid (person of Tawheed) is not thrown into the Fire as the disbelievers are thrown, nor does he remain within it in the way that the disbelievers remain. So if the persons Tawheed is complete – along with his sincerity for Allaah in it, and he stands with all of its conditions with his heart and his tongue and his limbs; or with his heart and his tongue at the point of death; that then necessitates forgiveness for that which has come to pass from sins – all of them.
Along with his being prevented from entering the Fire altogether, so whoever actualizes the statement of Tawheed with his heart, and so expels from it everything other than Allaah by way of love and veneration and exaltation and awe and reverential fear and hope and reliance; it is then that his sins and wrongdoing will burn – all of them – even if they were like meerschaum (the froth upon the sea). For perhaps it may turn them into good deeds as has preceded in the mention of regarding the changing of bad deeds into good deeds.
For indeed this Tawheed; it is the greatest remedial solution (elixir), thus if a tiny particle of it were to be placed upon the mountains of sins and wrongdoing; it would turn them into good deeds.”
(Taken from: Jaami’ al-‘Uloom wal-Hikam p.562-563)
 The reason for this is explained by Ibn Al-Qayyim (rahimahullaah) who said: “Pardon is granted for the people of pure sincere Tawheed who do not contaminate it with shirk, the like of which is not pardoned for the one who is not as such. So were the Muwahhid – who did not associate anything in worship with Allaah at all – to meet his Lord with the equivalent of the earth (in the measure of) in wrongdoing; He will come to him with its equivalent in forgiveness. This does not occur for the one who’s Tawheed is deficient and has contaminated it with shirk, for indeed; pure unadulterated Tawheed which has not been contaminated with shirk – then no sin remains with it, since it comprises of love of Allaah and exaltation of Him and veneration of Him and fear of Him and hope in Him Alone – that which necessitates washing away sins, even if they be equivalent to the earth (in vastness). Hence the impurity is a hindrance; and yet its repellent is strong – and so does not hold out with it.” (See: Ighaathatul Lahfaan vol 1 p.106)