I believe that there is a direct relationship between the Holy Roman Empire and the Roman Church beginning with Constantine made Christianity the State religion.
Slavery existed in various forms is not only in Rome, but in Christian nations up until recent history
Christianity - The push for human rights developed because of Christianity. Christianity alone stubbornly insisted that each individual was of inestimable value, and the idea revolutionized the world.
The idea of the equality of human beings was introduced in the very first Christian documents. "There is neither Jew nor Gentile, neither slave nor free, nor is there male and female, for you are all one in Christ Jesus" wrote St. Paul. Not a single Greek philosopher or Roman Stoic proposed that human beings were equal. In the ancient world, up to one third of the population were slaves. Slaves were regarded as less than human. Pliny had over 500 household slaves and many more thousands on his estates.
The only religion that stopped slavery was Christianity and Christianity ended slavery not once, but twice. Christians began attacking slavery even as Rome declined. Augustine thundered that slavery was “sin” even as the ancient world was breaking apart and Vandals were about to conquer Augustine’s city of Carthage. The church at Hippo once organized a raid and freed 120 slaves.
Slavery was first banned in Europe in the Dark Ages. Serfs replaced slaves, and, unlike slaves, they had rights, rights that grew stronger with every passing century. Furthermore, the nobility had obligations to the serfs. By 1520 Father Fransisco de Vitoria wrote that "all men are equally free; on the basis of natural liberty...their right...to life, to culture, to liberty." Even the greatest saint was equal to "a sinner or pagan in regards to natural rights." Vitoria has been called the father of international human rights.
In the Middle Ages, whole religious orders were formed to pray for, and collect money to buy back slaves from Islamic countries. This is stunningly different from the history of every other religion.
Slavery began again, in 1435 in the Canary Islands. The pope immediately issued the bull “Sicut Dudum” which proclaimed that anyone who owned, sold, or transported slaves was excommunicated, likely facing hell. The bull “Sublimis Deus” in 1537 reiterated the excommunication. Thousands listened; other thousands upon thousands did not. Human nature is flawed, sick with sin.
But soon Protestant abolitionists like Samuel Sewall and Jonathan Edwards sparked movements to end slavery forever. Quaker and Evangelicals organized antislavery movements that ignited the world. Hundreds of thousands and then millions rallied to the cause.
Alas, slavery exists today. According to the Global Slavery Index India has 11 million slaves. Afghanistan, Turkey - dozens of countries where people are still in bondage