God accepts the sacrifice (for Him) only from those who have taqwa (fear for God in every action)!

Verses 27-28 of Chapter 5:

And recite to them the story of the two sons of Adam in truth; when each offered a sacrifice, it was accepted from the one but not from the other. The latter said to the former: "I will surely, kill you.'' The former said: "Verily, Allah accepts only from those who have Taqwa. "If you do stretch your hand against me to kill me, I shall never stretch my hand against you to kill you, for I fear Allah; the Lord of all that exists.''