Really? of the 10 Commandants, numbers 5-10 deal with how man should relate to man - which of these do the Japanese reject? None. They acknowledge all of these because they intrinsic to every man's heart.
"Different society, different morals"
And what is the difference post 1945? It's the introduction of western Christian culture.
"Slavery was not considered immoral by many, if not most Christians for centuries."
It was Christianity, based on Christian morals, which led the fight to abolish slavery in every western society. Polygamy has immoral since Adam and Eve and this was reiterated in the 10 Commandments and the teachings of Jesus. It was always immoral. That's why God finally wiped out Israel and the Jews spent 70 years in Bablylonian captivity - they refused to follow God's commands. The inability of individuals or even societies to live up to the fixed morals of the creator does not mean the absence of the same. Every polygamist in the Bible suffered for it.
If you are going to make statements of faith as statements of fact you better have some hard evidence to prove them.
I do have hard evidence. Although every epistemology must start with certain unprovable axioms, (even empiricism) even these can be corroborated by human experience itself. The evidence is there for anyone who truly wants to look at and for it.
Raise a human being from infancy in your back yard, give him water and food and shelter and let him fend for himself. He will lack all social normals of modesty, morals, language, reading and writing skills, etc. He will have no clue where he is, how he came about, or anything we know; he will be a beast.
There is nothing intrinsic in our nature that even approaches the Ten Commandments. Everything we know has to be taught. And the society, as a communal consensus, dictates what is being taught as good and what as bad.
For exmaple, Islam teachs that mercy killing of one's daughter is an "honorable" thing to do for a father. Judaism taught, with the authority of God's own words, that a disrespectful child is to be stoned to death.
The only reason the last five commandments are kept is because it has to do with property and rights, strictly societal values. Commandments 5 to 10 simply reflect universal societal )materialistic) values, and the Japanese share them with us, but they see them as necessary for a functioning society, and not something intrinsically moral any more than the speeding laws. The Japanese society functions on the concept of empowerment, nor intrinsic morality.
Besides, why only the Ten when, as a believer, you must believe that God wrote 613 of them in all and dictated them verbatim to Moses?
How did Christians decide the rest weren't divine enough for them to follow?
It was Christianity, based on Christian morals, which led the fight to abolish slavery in every western society
Oh, rubbish. It was humanism and the Age of Enlightenment that contributed to the elimination of slavery, not Christianity. Christianity had no problems with slavery because it biblically sanctioned.
Polygamy has immoral since Adam and Eve and this was reiterated in the 10 Commandments and the teachings of Jesus. It was always immoral
Really? Then why is is allowed in the Bible (along with slavery)? And where is it "reiterated" in the Ten Commandments? And where did Jesus say one can't have more than one wife?
That's why God finally wiped out Israel and the Jews spent 70 years in Bablylonian captivity
They were worshiping idols (that's the only thing that would make the Old Testament God "lose" it); that's why, not because they were practicing polygamy or slavery.
I do have hard evidence. Although every epistemology must start with certain unprovable axioms, (even empiricism) even these can be corroborated by human experience itself
Epistemology, empiricism? Philosophy doesn't run the world. It's hot air unless it can be tested against the real world, regardless how you "feel" about it.
As for empiricism, human experience, like human memory is about as unreliable as it gets.
The evidence is there for anyone who truly wants to look at and for it
What evidence? Your axiomatic a priori assumptions based on how the world "feels" to you?