The third cause of forgiveness is monotheism and this is the greatest cause as whoever missed this has no forgiveness and whoever came with it, came with the greatest cause of forgiveness.
The Exalted said,
"Allah does not forgive he who associates anything with him and forgives less than that to whoever he wills." [Quran, 4:58]
Whoever comes with monotheism with the like of the earth, its likeness full or the like of it full shall receive from Allah (the Exalted) forgiveness the like of it but this is according to the will of Allah (Mighty and Majestic); if he wills, He forgives and if He wills He takes him to task for his sins then punishes him but he does not enter the hellfire forever rather he comes out from it and enters paradise.
Someone said, "The monotheists are not received in hell like the disbelievers are received and they do not remain in there like the disbelievers do."
If monotheism of the servant is perfect and it contains sincerity for Allah (the Exalted) in it, all the conditions of the heart, tongue and limbs or heart and tongue are met, before death, then it necessitates forgiveness of all his previous sins and he is prevented from remaining in hell, forever.
Whoever has realised the words of monotheism has removed love of everything from his heart except Allah, his magnitude, his majesty, awe, fear, hope and divine providence.
In that time his sins and errors all vanish, even if they are like the foam of the sea. And perhaps before it was good actions, as we have mentioned that they were changed from wrong actions into good actions. As monotheism is the greatest destroyer when placed before a mountain of sins and errors thus changing them into good actions.
As in Al-Musnad and others, Umm Hani reports that the Prophet (may Allah bestow peace and blessings upon him) said,
"Saying there is no deity except Allah; does not leave sins or actions preceding it." [Imam Ahmed, Al-Tabarani in Al-Kabir and Ibn Majah]
In Al-Musnad reports that Shadad Ibn Owais and 'Ubaydah Ibn As-Samit said that the Prophet (may Allah bestow peace and blessings upon him) said to his companions,
"Raise your hands and say there is no deity except Allah." We raised our hands for a moment then the Messenger of Allah (may Allah bestow peace and blessings upon him) put his hands down and said, "Allah is worthy of all praise. O Allah who sent me with the truth of these words, ordered me by it and promised Paradise to me for it and he does not break his promise." Then said, "Rejoice, as Allah has forgiven you." [Imam Ahmed, Al-Bazzar in Kashaf Al-Astar, Al-Tabarani in Al-Kabir, Al-Hakim & Al-Dawlabi in Al-Kani]
As-Shibli said, "To trust the world is to be set ablaze by fire and become ash and destructive smoke. To rely on the afterlife is that one is set ablaze by light and becomes gold reddened and becomes beneficial. And to depend on Allah is to be ablaze with the light of monotheism and become like priceless jewels."
When the fire of love hangs in the heart it burns everything in it except (love of) the Lord (Mighty and Majestic) so the heart becomes pure, from that moment, from dirt and monotheism is soundly planted.
"My heavens cannot encompass me nor my earth but the heart of my believing slave can encompass me." [Ahmad]
"Longing is choking to them; shining and burning in passion; love is thrown into the depth of the sea; so take by Allah the hand of the drowned; permitted to me is your love in preoccupation; take from me every promise and agreement."