A simple question for theists

[RoadRunner79]

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...killing an innocent person goes against Natural Law. Meaning it goes against the way our world works, because that is how God created our world to work. Thus we have an inherent understanding that directly killing an innocent person is wrong.

Exactly that. At the same time, The OP's hypothetical is that you believe the person is 100% innocent. As fallen creatures, we must admit to our own limitations - there is no 100% and so while you may believe the person is innocent isn't it also possible that you are not in command of all the facts and must trust that God's judgment is just?

As for Abraham, remember that God did't actually have Abraham kill his son, but was merely testing him. Furthermore, I personally find it hard to believe that story actually happened in the literal way it was written...Did God's voice sound from the sky and tell Abraham to kill his son, and then tell him not to? My guess would be no...as a Catholic, I am free to take a literal or allegorical approach to the Old Testament stories. I tend to lean to the latter. 

As you know, I feel people should be careful to discern between what should be taken figuratively as opposed to literal versus. In this case, Gen 22, figurative elements are clearly present, however, I see no reason to take this story as pure allegory.

Lastly, Christianity is defined by the New Testament.

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Sorry, but I must protest. The NT cannot be divorced from the OT. As per my earlier citation, on the road to Emmasus, the resurrected Christ opened the eyes of his disciples to show them how the Law and Prophets spoke of Him. Or in John 5:46 where Jesus says "if you had believed Moses, you would believe me, for he wrote about me..." Or the story of the eunuch reading Isaiah and not understanding it until Peter explained it to him.  Christianity is defined by both the NT and the OT together as a seamless narrative.

I gave kudos mostly for that last part. While I don't believe that the doctrine of inerrancy is quite necessary for the Christian faith. You cannot remove it from it's history. In is one story, with one God.