The Bible is a progessive revelation. Abraham was very early, before there was even a Jewish nation, well before Moses and the Law.
I cannot answer "why" God seems to have been testing Abraham. It certainly was not because God needed to know something about Abraham.
Mankind has been fighting against God sice the very beginning. Why would anyone think that God is under some sort of obligation to serve mankind. He is the Creator; we are the creation--VAST difference. Paul discusses this in his letter to the Romans--Does the clay ask of the Potter, " Why have you made me like this?". The original Greek is very strong--that the question is completely out of order==Clay does not make demands on the Potter.

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1982 300SD " Wotan" ..On the road as of Jan 8, 2007 with Historic Tags