A keystone in Jewish and Christian belief is faith in God. The supreme example of faith is the Bible story where God commands Abraham to kill his own son Isaac as a sacrifice to God. Abraham agrees, and is praised for placing his loyalty above morality.
An atheist asks: "How can you believe in a God that would ask Abraham to kill his own son as a sacrifice? Should blind loyalty to God lead a man to commit evil, inhuman acts? And what sort of a God is that?"
Makes you think of the Nazis, doesn't it? And raises the spectre of Pastor Jim Jones telling all those cult members to drink poison in Guyana. If God tells you to kill someone, should you do it?
It's a great question. But, like any good movie plot, there are several twists.
First, coming from an atheist this question is disingenuous - not entirely honest. An atheist lives in a world which is a random pile of atoms. Atoms have no morals. So it is illogical for atheists to recognize the existence of moral absolutes! They believe Man is the greatest thing in the universe - so who invented good and evil?
There are of course acts which most men agree are distasteful (like murder), or unprofitable (like killing your pension - your only support for your old age), or unkind (like killing your own son), or bad for the human race (like killing off all your descendants which will be "like the stars in the sky").
But in an atheist world how can anything be intrinsically evil? - there are no outside standards, only human opinion. In that world, "evil is in the eye of the beholder". So this atheist is adopting our standards to argue with us!
But it's still a good question. Why did God tell Abraham to kill Isaac?