One can perhaps try to generalize Plato's argument about Greek gods always quarreling, etc. into trying to analyze and find logical inconsistencies in these books. Indeed people have noticed these, e.g. in the case of Quran sone suras that seem to contradict earlier ones, but they have explained these away with various ingenious devices, because they are pious.
So, we arrive at the analytic situation where people are pious because they follow the book and they follow the book because they are pious, which to me is equivalent to the Euthypros's definition.
I don't know that saying "piety is what the Christian God says is piety" is useful, because in that case it is your responsibility to have sex with your brother's wife if he can't conceive a child, to not kill your slaves unless they've damaged your property and to beat your wife only with a thin stick, and not a thick one.
The point is that, in any religion, someone picks and chooses. That may be you, or it may be the scholar/marketing person who wanted the message to be relatable, up to date and consistent. So the question becomes, when the choice comes, what is chosen?