Haw!!!
Corpusdei, that is an EXCELLENT question, and one that always bothered me as a child being bullied into the Jehovah's Witness religion.
However, when I got away from the cult, I did a little research and found out several things...
First of all, keep in mind that 'Abraham' was supposedly living amongst the "Philistines" - but more likely among the Canaanites - prior to this attempt to sacrifice 'Isaac'....
Now, here's the interesting part...
The "Canaanites" weren't actually wiped out by the Israelites - if indeed the Israelites were ever a real threat to them. Apparently, changing forces in the area caused the Canaanites to migrate across the Mediterranean - and one of the places in which they settled was CARTHAGE, North Africa.
The Carthaginians became a force to be reckoned with, even presenting a threat to the Roman empire. The "Punic Wars" were basically caused by the Roman attempts to defeat and obtain Carthage as a base of operations for the rich North African farmlands [the area was greener, back then....]
Remember Hannibal crossing the Alps?? His name - Hanni bal - is basically "Hanni - Ba'al" ... And WHO was it that worshipped Ba'al?? Remember??
Anyway, the Romans claimed that the Carthaginians practiced - are you ready for this?? CHILD SACRIFICE....
Now, let's segue back to ol' 'Abraham'...
I strongly suspect that 'Abraham' was initially worshipping the "Philistine" or Canaanite gods... Apparently, these gods required child sacrifice.
So, here's old 'Abraham', going up onto the mountain to practice child sacrifice, when "suddenly" a 'new' god - or a variation upon the gods 'Abraham' knows - initiates a "new" practice... And you'll notice WHY, in Genesis 22:17... "I will indeed bless you and make your offspring as numerous as the stars in the sky and the sand on the seasore. Your offspring will possess the gates of their enemies. ..."
I suspect that canny old 'Abraham' realized that he wasn't going to become numerous if he continued worshipping the "Philistine" or Canaanite gods... So he "SUDDENLY" got this message from a "new" god... And saved his legitimate son - the one born of the woman with the tribal connections that WEREN'T "Philistine" or Canaanite, [edited... instead of the slave woman with NO practical tribal connections and therefore no real opportunities for political alliances with powerful tribal groups...] and therefore more likely to produce a large fighting force... Pragmatic old sheepherder, 'Abraham'...
I was always amazed at the idea of [male] child sacrifice, as it is a recipe for disaster, from a military standpoint Any nation, civilization, or tribal group needs to have a LARGE population of young males to successfully wage warfare. Sacrificing children - especially the male children - seemed counter-intuitive. If the Carthaginians HADN'T practiced child sacrifice, they might have been successful in fending off Roman incursions...
Going back to the Romans.... Many archaeologists and scholars of Roman and North African history discounted the Roman tales of child sacrifice as mere propaganda. However, recent discovery of a Carthaginian cemetary with multiple child skeletons - very young, and mostly male - has changed the archaeological viewpoint of the Carthaginians/Canaanites...
It seems that the Canaanites - Carthaginians - might have practiced child sacrifice, after all...
And it only made the most practical sense for 'Abraham' to have that sudden 'revelation', thereby saving his younger - NOT THE FIRSTBORN - son - which, as I mentioned above, gave 'Abraham' tribal, political alliances that he could actually use...
Zid