As I don't believe the countries you mentioned were founded on Christianity, they were mostly founded on European feudal, hereditary kingship and aristocracy - as far from Christianity as can be.
As you stated, the USA was not really founded on Christianity either, but on secular values. As I have stated, it is the New England heritage of righteous Christianity that I have been alluding to,- those states never allowed slavery and were its greatest enemies.
It was precisely to the extent of the lack of Christianity in the US (after, lets remember, the original Christian values of the Northern states had been replaced by more secular ones), that allowed the new US republic to turn a blind eye to slavery.
And, it was God's righteous wrath on slavery that saw the South defeated and humbled in the war between the states. A terrible misfortune as in every other way the South's cause of secession was righteous and good. But the Kingdom of Jesus Christ is incompatible with slavery. Indeed it was His influence that originally removed the practice of it from Europe as Christianity spread following the dark ages. As you say, it crept back in later. Not by Christianity, but by the lack of it.
Obedience to God is resistance to tyrants.
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