I think you have a loaded question, intrinsically tied to cultural imperialism, and designed to support your own beliefs.

Europe, as the main world power for centuries, had its fingers everywhere on the planet. Yes, they were Christian. But, so what? These same Christians were also the ones who put the trans-Atlantic slave trade together in the first place.

You need to ask yourself how is it that the same Bible which influenced the abolitionists somehow did not prevent Christians during the previous 1,500 years from banning slavery, or from owning slaves?

You question about "what caused slavery to decline" is addressed at https://en.wikipedia.org/wiki/Abolitionism and links from there. You can read that, yes, there was a religious component:

> An abolitionist movement only started in the late 18th century, however, when English and American Quakers began to question the morality of slavery. James Oglethorpe was among the first to articulate the Enlightenment case against slavery, banning it in the Province of Georgia on humanist grounds,

Note that during this Quakers were considered heretics and many of their views blasphemous. But even Quakers, when they first came to North America, though that slavery was acceptable. Quoting https://en.wikipedia.org/wiki/History_of_the_Quakers#Slavery... :

> Most Quakers owned slaves when they first came to America; to most Quakers "slavery was perfectly acceptable provided that slave owners attended to the spiritual and material needs of those they enslaved."[25] 70% of Quakers owned slaves in the period from 1681 to 1705; however, from 1688 some Quakers began to speak out against slavery until by 1756 only 10% of Quakers owned slaves.

Again, you have to ask yourself why it took devout Christians in the 1700 so long to see that the Bible is actually against slavery?

There's a clue in the above text. Note the "Enlightenment" in there? If you read https://en.wikipedia.org/wiki/Abolitionism_in_the_United_Sta... you'll see:

> In the same period, rationalist thinkers of the Enlightenment criticized slavery for violating human rights. A member of the British Parliament, James Edward Oglethorpe, was among the first to articulate the Enlightenment case against slavery. Founder of the Province of Georgia, Oglethorpe banned slavery on humanistic grounds. He argued against it in Parliament and eventually encouraged his friends Granville Sharp and Hannah More to vigorously pursue the cause. Soon after his death in 1785, Sharp and More joined with William Wilberforce and others in forming the Clapham Sect.

Yes, what's new is the contribution from Enlightenment thought, which is based on another non-conformist (and non-Christian) faith; Deism. Quoting now from https://en.wikipedia.org/wiki/Christian_humanism#Enlightenme... :

> The Enlightenment of the mid-18th century in Europe consolidated the separation of religious and secular institutions that has led to what some consider to be a false rift between Christianity and humanism. But while the Enlightenment crystallized humanism as a distinctly secular, liberal philosophy, it did have sectarian roots that reached back to early 18th-century England.[3] There rationalists known as ‘Deists’ rejected traditional theology and clericalism in favor of ‘natural religion’. Non conformists, they preferred to sidestep the churches and seek God personally by way of reason and innate moral intuition.

You can see the ties to Quaker thought in that description. The page also says:

> Perhaps the most valuable contribution of this liberal Christianity is that it gave rise to the first British movement for the abolition of slavery, which was founded by the Quakers in the late 18th century. However, it was the Evangelical Christian humanism of William Wilberforce (24 August 1759 – 29 July 1833) that led to the successful abolition of the slave trade.

Yes, it wasn't until the humanism of the secular Enlightenment, coupled with the non-conformist beliefs of the Quakers, that the Evangelical Christians started to think that maybe, just maybe, even non-Christians shouldn't be slaves.

It wasn't the Bible that kicked this off this change in how to understand Christianity, it was the Enlightenment.