Abdullah narrates that he asked the Messenger of Allah ﷺ, “Which sin is the greatest?” He replied: “Your considering others equal to Allah, while He has created you.” I asked: “After that, which is the next?” He replied: “Your killing your children for fear that they would share with you your food.” I asked: “Which one after that?” He replied: “Your having adultery with the wife of your neighbour.”
This hadīth mentions three major sins. The greatest among them is considering others equal to Allah. This is called polytheism, which means to believe that someone other than Allah possesses divine attributes, or he is also worthy of being worshipped like God. The Qur’ān calls polytheism the greatest evil also. This hadīth also provides a solid argument in favour of monotheism: Allah is our Creator, then how can one who is simply a creature, and not the creator, have the attributes of the Lord Creator? Isn’t one’s act of making someone else one’s Lord an act of ingratitude to the Creator and disloyalty to Him? And who can be a greater sinner and thus criminal than one who is ungrateful and disloyal to his own Creator?
After polytheism, the next greatest sin that has been mentioned here is the killing of children. In the pre-Islamic days Arabs used to bury their children, particularly girls, alive, for fear of poverty. This is the most heinous act.
Similarly, fornication is a major sin. When it is committed with the wife of a neighbour, its wickedness and intensity is increased.