Ok, so under the assumptions that God is good and his commands are for the best, here's how I answer the first two:
(1) If God commands you to kill your own child, then you should do it.
True
(2) If God commands somebody to kill their own child, then they should do it.
True
Things aren't as easy for the next two questions.
(3) If you sincerely believe that God has commanded you to kill your own child, then you should do it.
Here's what a friend said about this question. I know the level of evidence I would need to sincerely believe that God had commanded me to kill my own child - it would be really,
really high. This response amounts, I think, to saying that I would only have that sincere belief in the case that God did in fact command me to kill my own child. And assuming God is good and all then, then If that's right, then we should borrow the assessment of question 1 and say, true. At least,
I should say 'true', because I believe the level of evidence
I would demand to accept the command would be extremely high. I don't know about
you ;)
(4) If somebody sincerely believes that God has commanded them to kill their own child, then they should do it.
But here things get a bit more complicated. It's tempting to say at first glance that believers ought to accept (4). But I'm not so sure, and it seems to me that what I've said about (3) comes into play. The concern here is that some people might form sincere beliefs on much lower standards of evidence, and for those people, even if I think (3) is true, I want to say that (4) is false; that is to say that it isn't the case that if somebody has that sincere belief, then they should do it. Specifically, if that belief wasn't formed on the basis of some sufficiently high standard of evidence, then they shouldn't. On this analysis, (4) as it's worded should be regarded as false.
Before I go further, I'd like to know what you guys think about what I said about (3).
Also, Jersey Girl, I'm not quite sure how to challenge you. You're answer is that in the case where God in fact does command you, and God is good, you should not do what God commands. I'm not sure why you think you shouldn't. It might help me if you would explain why you believe in this case that you should not do what the good God commands.