Divine Oneness (Tawhid) is the Key to Heaven

God has forbidden Heaven to those who commit shirk, it being the worst of the three kinds of injustice. None who associate partners with God will enter into Heaven. The people of tawhid [‘Divine Oneness’] will do so, because tawhid is the key. The door shall not open for anyone without this key, or for anyone who comes with a key that lacks its teeth. The teeth of this key are prayer, fasting, alms, pilgrimage and sacred combat. They enjoin justice upon people and prohibit wrong; they speak truth and honour trusts; maintain family relations and treat parents well.

Anyone who acquires the right key in this life, the key of tawhid, and then gives it teeth by [keeping God’s] commandments, will come on the Day of Judgment to the door of Heaven with the only key that opens it. He will not be kept out unless sins, errors and burdens remain with him, the traces of which had not been removed through repentance and the search for forgiveness [while] in the world. These will bar him from Heaven until they have been cleansed away. If standing before God with all its terrors and distress will not accomplish this, hell cannot be avoided if the person is to be rid of what foulness remains in him, if his defilement is to be cleansed—until he is brought out and placed in Heaven. For Heaven is the abode of the pure, and none shall enter it except they. God says, ‘The pure ones whose souls the angels take, to them it will be said, “Peace be upon you. Enter Heaven”’(XVI:32); and, ‘Those who revere their Lord are conducted to Heaven in ranks, until they come unto it and its doors are opened, and its guardian say to them, “Peace be upon you, you who are pure. So enter herein forever…”’(XXXIX:73)

Hell is the abode of [all] that is corrupt in word and deed, food and drink; it is the abode of the two impurities, ‘That God may separate the corrupt from the pure. The corrupt He shall place one upon the other, heaping them all together and consigning them to hell. Such are the losers’—in the words of God (VIII:37). He will gather the corrupt and heap them high, as if to be carried away; He will place them in hell along with its denizens, and there will be nothing therein except what is corrupt.

Just as people will be of three levels—the pure [al-tayyib] with nothing corrupt in them, the corrupt [al-khabith] with nothing pure and those in whom both are mixed—so, too, will their abodes be three. There will be the abode of the fully pure and the abode for the fully corrupt—both eternal. But there will be the abode of those in whom purity and corruption are mixed. That abode, which will not last for them, will be a house of correction. But no one who believes in God’s oneness and has undergone punishment will remain in hell. Rather, after they have been punished to the measure earned, they will be taken out and placed in Heaven. And then naught will remain save the abodes of the fully pure and the fully corrupt.

- Ibn Qayyim al-Jawziyya, The Invocation of God (Al-Wabil al-Sayyib min al-Kalim al-Tayib)