Why did God command Abraham to sacrifice Isaac?

Did he really sacrifice him, or when he was about to sacrifice him, did God put a lamb in his place? Isaac died a sacrificial death? Genesis 22 (NIV) > 1 Some time later God tested Abraham. He said to him, “Abraham!” “Here > I am,” he replied. 2 Then God said, “Take your son, your only son, > whom you love—Isaac—and go to the region of Moriah. Sacrifice him > there as a burnt offering on a mountain I will show you.” > > 12 “Do not lay a hand on the boy,” he said. “Do not do anything to > him. Now I know that you fear God, because you have not withheld from > me your son, your only son.” If Abraham really sacrificed Isaac, isn't it a cruel act, just to show Abraham's faith?