For those that may not know the scripture reference, it is Genesis 22:1-12. This is one of those key parts of scripture that atheists and, frankly, quite a few Christians always get wrong. The entire passage is a foreshadowing of God offering his only Son to die for the sins of humankind. (Genesis 22:2, John 3:16)

On top of that, Abraham knew that he would not ultimately have to kill Isaac, or if he did that God would raise him from the dead. In Genesis 13:16 God promised Abraham, "I will make your offspring like the dust of the earth, so that if anyone could count the dust, then your offspring could be counted." As Isaac was his only son, there would be no way for this to come to be had Isaac died that day (or if he had, was not raised from the dead.) Ref Hebrews 11:17-29, "By faith Abraham, when God tested him, offered Isaac as a sacrifice. He who had embraced the promises was about to sacrifice his one and only son, even though God had said to him, “It is through Isaac that your offspring will be reckoned.” Abraham reasoned that God could even raise the dead, and so in a manner of speaking he did receive Isaac back from death."