Abdullah relates that he asked Allah’s Messenger ﷺ: “Which sin is the gravest in the eye of Allah?” He (the Messenger) replied: “That you associate a partner with Allah (although) He has created you.” He again asked: “What is the next?” The Messenger replied: “That you kill your child out of fear that he shall join you in food.” He again asked: “What is the next?” The Messenger replied: “That you commit adultery with the wife of your neighbour.”
(Sahih Muslim)
The hadīth names three gravest sins. The most grievous sin is associating partner with God. The reason is that man is created by God and it is the height of ingratitude on his part that he associates other partners with his Creator. Moreover, this act also lowers his own dignity.
It is also a kind of polytheism that man assumes the role of a nourisher and thinks that he is responsible for the nourishment of his children. The practice of killing children and other methods of population control are in fact denial of the Lord’s attribute of nourishing. The modern craze of birth control is based on a false concept that the resources God has created to maintain humanity are far meagre as compared to the ever-increasing population. While the fact remains that God is very Bountiful and Merciful and the means He has provided are far more vast and rich than the growth of population.
Committing adultery with the wife a neighbour is denial of the Ever-Vigilant Eye of the Omnipresent Lord.