إِنَّ هَٰذَا ٱلْقُرْءَانَ يَهْدِى لِلَّتِى هِىَ أَقْوَمُ وَيُبَشِّرُ ٱلْمُؤْمِنِينَ ٱلَّذِينَ يَعْمَلُونَ ٱلصَّٰلِحَٰتِ أَنَّ لَهُمْ أَجْرًا كَبِيرًا

The Quran calls upon all human beings to accept the Oneness of God, or tawhid, that is, belief in the one God and surrender to His will. It can truly be said that no other matter can be more correct, more reasonable and more natural. Undoubtedly, the Oneness of God is the greatest reality and also the greatest truth. This position of the Oneness of God necessarily makes it the standard by which all human beings should be tested and the basis on which some should be treated as right and others as wrong, some as successful and others as failures. But, in the present world this standard is seemingly not upheld and the testing of human beings is not, apparently, done on this basis. But this is due only to God’s hidden way of applying His law of testing. For individuals, death, and for the general mass of the people, the Day of Judgement are the final limits of the test period. As soon as this limit is reached, human beings will be separated into two different groups: those who believed in one God in the full sense, and met all the demands of that belief, will find themselves in paradise, while those who did not, will find themselves in hell.

— Source: Tazkirul Quran (Maulana Wahiduddin Khan) via Quran.com (17:9)