Consider this list of indictments from 2 Kings 21:5-6. "In the two courts of the temple of the Lord, he built altars to all the starry hosts. He sacrificed his own son in the fire, practiced divination, sought omens, and consulted mediums and spiritists. He did much evil in the eyes of the Lord, arousing his anger."
Ignoring the weirdly casual reference to infanticide, it struck me that when Abraham did this, he established the faith for all time. Of course, God called it off, but wasn't it required that Abraham intended to go through with it.
To me, this means that the substance matters. Why was it bad what Manasseh did? Because the god to which he was sacrificing wasn't real. And, my point is precisely that from a rational analysis there is just no way to distinguish.