They even quote the passage from Revelation. They must be pretty smug if they don't expect their pigeons, uh, I mean followers to fail to notice the general abolition of slavery. With the current political climate, it would be possible to unite the world peaceably without enforcing the no-slave rule universally. (It probably wouldn't be possible to unite the world peaceably at all.) The passage was written by a person (not a god) who expected all this to take place within his lifetime. Back then, slavery was common. But, it is effectively a prophecy that slavery would continue in the developed world until the "end of days." As such, it fails.
Incidentally, I say "in the developed world" because in any merger between developed and undeveloped lands, the rules of the developed land are what take hold. Of course, I am sure that all True Christians™ are praying for a return to slavery. For those who protest and say they do not want a return to slavery, it simply means that they are "false converts."