'Abdullah reported: I asked the Messenger of Allaah (sallAllaahu alayhi wa sallam): Which sin is the gravest in the eye of Allaah? He (the Prophet) replied: That you associate a partner with Allaah (despite the fact) that He has created you. He (the reporter) said: I told him (the, Prophet): Verily it is indeed grave. He (the reporter) said: I asked him what the next (gravest sin) was. He (the Prophet) replied: That you kill your child out of fear that he shall join you in food. He (the reporter) said: I asked (him) what the next (gravest sin) was. He (the Prophet) observed: Then (the next gravest sin) is that you commit adultery with the wife of your neighbour. (Muslim)

Lessons to learn from this Hadith

1. Shirk is the most gravest of sins.
2. Allah will forgive any other sins if He wills. But Allah will not forgive the sin of shirk unless and until the person who committed shirk repents and totally stops it.
3. Killing own child is a another major sin. We cannot kill the child because of the fear of poverty.
4. The next major sin is committing adultery with neighbour's wife.