Abdullah relates that he asked the Messenger of Allah (peace and blessings of Allah be to him): “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 hadith names three gravest sins. The most grievous sin is associating partner with Allah. The reason is that man is created by Allah and it is the height of ingratitude on his part that he associates other partners with his Creator. Moreover, this act also lowers his dignity and status.
It is also shirk (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 increasing population. While God is very Bountiful and the means He has provided are far more vast and richer than the growth of population. Also, a person has every right to expect security from his neighbour that his trust would not be betrayed.