Rereading Abraham and Isaac at Moriah: I think some people misunderstand what happened and what God was doing. For those not aware of it, a pagan society has a lot of gods and spirits/forces that need to be placated. Some can be friendly, provided you do your part. One spirit will be satisfied with an offering of nuts, but another will only accept the nuts as a promise of a chicken later. The more important the spirit/god, the more important and expensive a sacrifice you must offer. (You wouldn't offer to take your boss out to dinner at McDonalds, would you?) While some groups (like the Aztecs) start going hog-wild with the sacrifices, generally a human sacrifice is only something the most important gods require, and then only on special occasions. The highest sacrifice is, of course, a son.

So what is God saying to Abraham with His first summons? "I am the kind of God who is powerful and august enough that only the best sacrifice is enough." And then the second announcement says "I don't require the sacrifice--I'll supply that--I require the devotion." In the culture Abraham lived in, this lesson would have been very clear. We have a little trouble seeing this, living in a culture that doesn't worship the same ways, but Abraham understood.