Abdullah relates that he asked the Messenger of Allah (peace and blessings of Allah be to him), “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 of 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.” (Bukhari)
Considering others equal to Allah means to believe that someone other than Allah possesses divine attributes, or he is also worthy of worship like God. This is polytheism and the Holy Qur’ān calls it the greatest oppression (zulmun azeem). This hadīth argues why polytheism is the greatest sin. ‘While He has created you’, how can anyone or anything else have the attributes of the Lord Creator and deserve your worship? The act of making or even considering anyone or anything other than One God is in fact an act of ingratitude to your Creator and disloyalty to Him. The sin next to polytheism is the killing of your children for want of food. In the pre-Islamic days, people used to bury their own kids, for fear of poverty. This is the most heinous act one can imagine.
Similarly, fornication is a very great sin, but when it is committed with the wife of one’s neighbour, its enormity is increased many times. After all, a neighbour is expected to safeguard the interests, and never to rob the honour, of his neighbours.