It is difficult to answer a question pertaining to such a wide ranging timespan as "in the past", so I will take the liberty to boil it down to a time and place when discussions about slavery where indeed very relevant.
During the eighteenth century, spurred on by the Enlightenment, a lot of discussion took place about what exactly constituted civil society. Where women allowed to be part, or foreigners, or even slaves? This was of crucial importance because being part of the civil society meant, in the general Enlightenment discourse, that you had civil rights. Obviously slavery is the negation of any of those civil rights, or even the acknowledgement of ones humanity. Thus slavery poses one of the most highly contested problems for Enlightenment discourse, even during the era itself. Nowadays we have scholars who are critical of Enlightenment discourse because of it's failure to deal with this problem of slavery, and maybe even consolidation of racist policy's. 1
Now for religion. It can be argued that the christian religion can be, and was used both as a legitimation for slavery and an attack against it. A large part of those critical of slavery where devout christians who argued that the love of christ should be universal and included those who were held as slaves. But equally the bible could be used as a legitimation; the old testament is clear about the early fathers (Abraham etc) holding slaves.
An interesting example of this juxtaposition was to be found in Virginia during the 1780's. Arguably one of the more important slave colonies in North America during the height of the Atlantic Slave trade. The methodist church banned slave holders from attending their congregation, in 1784, on the basis of universal charity. The methodists argued for a ban on slavery, as was already the case in the northern colonies where slavery was also banned due to the influence of Quakers who held the same views regarding slaves. But the slave holders argued, petitioning in 1785 against a measure that would provide manumission for slaves, on the same grounds. They held that "Christ while on earth and giving instructions for things necessary for salvation, did not forbid it, but instead left behind injunctions to regulate masters and servants". 2
Thus slavery, Enlightenment, christianity and the bible all contributed to a paradoxical time where there was talk of universal human values and rights while large portions of the population where living under the brutality of slavery. 3 Religion did play a part in the discourse that could be compared to discussions about same sex love and marriage in the present. But one has to keep in mind that conditions where very very different back then; what it meant to be a citizen changed drastically and then as now there are many different sides to the story. Even the institution of marriage, and views on homosexuality changed a lot. Thus implying that using religion and the bible as an argument in discussions about who and what should have a place in civil society is akin to using the same arguments to defend slavery and segregation is maybe stretching it a bit. As usual it's more complicated, which it always seems to be when comparing public discourses across several century.
Sala Molins, Dark Side of the Enlightenment, 2006
Frederika Teute Schmidt & Barbara Ripel Wilhelm, 'Early Pro-slavery Petitions in Virginia', William and Mary Quarterly, 30 (1973), 143-4
Dorinda Outram, The Enlightenment, (1995/2013), Chapter 6: 'When people are property', 67-83
One needs to understand that yes, the Bible was used to justify slavery.
And then one needs to consider that practically the entire movement to abolish slavery was based on Christian principles and spearheaded by Christians who dedicated their lives to the cause. William Wilberforce, a deeply religious man, spent his life dedicated to the abolition of slavery in the British Empire, and was successful. The Quakers were among the first to oppose slavery.......and the list goes on and on.
http://memory.loc.gov/ammem/aaohtml/exhibit/aopart3.html
http://www.nla.gov.au/selected-library-collections/anti-slavery-movement-in-the-united-states
Christianity was the engine of the abolition of slavery.
A clarifying question, are you talking about slavery and segregation in the context of the United States south or some other context?
Am I allowed to ask a further question? I don't know the rules on that.
How accurate is Uncle Tom's Cabin in its portrayal of the way people felt about blacks? Did people, including church leaders really teach that black people were inferior and subhuman? I wish I had read the book more recently so I could expand more on the question.
If I'm breaking the rules, forgive me.
It depends a lot on what you mean by "same way" since these issues are virtually opposite in their history. By that I mean up until its abolition in America, slavery had always existed, and the movement was to end it. Gay marriage on the other hand is new and the movement seeks to begin it.
Another complication is marriage in the Christian understanding is a sacrament, which is an ecclesiastical term meaning a very special thing you do in the presence of God. Maintaining slaves had no such association.
A third difference would be the actual biblical reasoning behind retaining slaves, and opposing gay marriage. Slavery is pervasive in the Bible, sort of passively acknowledged as a historical reality without necessary giving any value judgement against the actual practice itself. Regarding homosexuality, there is a relative dearth of material in which homosexual behavior is discussed, and the notion of a homosexual marriage would never have crossed the biblical authors minds. The big message the New Testament does give regarding relationships though, is that you really shouldn't have them, and in fact, shouldn't get married if you can help it. This was also a time when polygamy was normal among several other differences between marriages now and then. In short, slavery in the Bible will give you a lot of material to work with, gay marriage and homosexuality, you've got much less, and you're also dealing with a completely foreign sexual ethic even to relatively conservative Christians.
I am not a historian, just a student, but I took a course on the history of medicine and we talked about how the bible was used to justify the anti-smallpox inoculation campaign that happened in Boston in the early 18th Century (I believe the exact date is 1721). In this particular episode, Cotton Mather and Dr. William Douglass squared off and debated the merits of inoculation. Cotton Mather had gone ahead and inoculated a bunch of people against smallpox while Dr. William Douglass and the rest of Boston was opposed. The justification by the rest of Boston and the clergy in particular, was that smallpox inoculation would delay the judgment of God (aka death by smallpox) and that letting smallpox run its course was the only way to atone for sins.
This paradigm of God's judgment being wrought by disease persisted into the early 19th Century at least. Charles Rosenberg's book, The Cholera Years, documents how cholera in NYC was thought to only be able to affect the poor, decrepit and sinful.
Hope that illuminates a little more on different ways in which religion was used to justify other practices throughout history.
I assume we are focusing just on the Christian justification for slavery.
In the early days of Christianity savery was already an entrenched part of society. It has been a long time since I read up on it but during the time of St Augustine (circa 400 AD) was when the idea of original sin comes to be a part of doctrine. With this comes the idea of predetermination and Augustine links slavery to both of these principles.
It is with justice, we believe, that the condition of slavery is the result of sin. And this is why we do not find the >word 'slave' in any part of Scripture until righteous Noah branded the sin of his son with this name. It is a >name, therefore, introduced by sin and not by nature.
St Augustine, The City of God, 19: 15
As we can see Augustine justifies slavery as caused "by sin and not by nature", essentially meaning that in a world without sin there would be no slavery but since 'the fall' and the original sin this is not possible and therefore sin is inevitable.
With this idea comes the idea of predetermination, while Augustine accepted that people had free will he theorised that in a post-fall world without God's intervention man would inherently turn to evil. Therefore sin was essentially god's will for the unchosen. With regards to slavery he said this.
The prime cause, then, of slavery is sin, which brings man under the dominion of his fellow -- that which >does not happen save by the judgment of God, with whom is no unrighteousness, and who knows how to >award fit punishments to every variety of offence.
St Augustine, The City of God, 19:15
So essentially Augustine argues that slavery is a condition that happens because God doesn't will it not to. He does make it clear though that this is our fault though, had Adam and Eve not sinned then we would not have sin by nature and therefore would not have slavery.
In short, yes, the Bible was a very important tool for defending slavery in the United States. Actually, religion was used on both sides. One example I can think of right off hand would be Samuel Sewall's work The Selling of Joseph. This piece uses Biblical passages to denounce slavery, using passages commonly cited to uphold slavery to argue the case against slavery. The Bible was seen as a source of truth, and was used in justifying slavery in the early years of the US (this piece was published in 1700). Sewalls, and many others of the time, used the passages like a lawyer to defend their position. As for segregation, I cannot help.
Yes. In fact, this is literally how the Southern Baptist Convention was created. Traditionally, Baptists were in accordance with Methodists and Quakers in urging their congregations to manumit their slaves. Baptists freely welcomed slaves into their churches and even allowed them to become preachers. The denomination which would become the Southern Baptists decided against challenging the southern status quo and interpreted the Bible as supporting chattel slavery. Slaveholders were encouraged to behave as father figures while slaves were taught to know their place and obey their owners. The justification for this involved lengthy exegesis from a variety of concepts found in Scripture but the three main points were:
Well into the Civil Rights Era, the Southern Baptists ignored and in some cases directly opposed the enfranchisement of Black Americans. They ardently defended slavery, segregation, and white supremacism. The SBC didn't formally renounce these views until they officially released a resolution on their 150th anniversary on June 1995.
All of this began from Baptists in the northern states refusing to support slave-owning missionaries. Of course, the south being what it was, you couldn't throw two rocks and not hit at least one slave owner. Baptists in the southern states felt they were being unfairly discriminated from funding because of this, so they formed their own denomination and retooled their theology to be more palatable to the local demographic.
At least some decision-makers used religious justifications to support anti-miscegenation laws (which I suppose isn't technically slavery or segregation, but probably related to the general thrust of your question).
As you might know, Loving v Virginia is the US Supreme Court case which struck down laws prohibiting interracial marriage.
I think the words of the Virginia trial judge (ie the lower court ruling, which was upheld in appellate court, but overturned by the Supreme Court) provide some insight into this.
In explaining why he was upholding the ban Justice Bazile wrote:
Almighty God created the races white, black, yellow, malay and red, and he placed them on separate continents. And but for the interference with his arrangement there would be no cause for such marriages. The fact that he separated the races shows that he did not intend for the races to mix.
Yale professor and Civil War scholar David Blight also touches on this question briefly in one of his Civil War lectures. The entire series is worth watching, but lecture #3, starting around 33:00, touches on the Biblical justification for slavery.
Blight observes that defenders of slavery would say "[slavery's] venerability is it's own argument, some say: 'It's always been around. Every civilization has had it. All those Biblican societies had it.'" Sadly, I still hear this argument all the time from people trying to downplay the horrific nature of American slavery.
I have in my possession The Pro-Slavery Argument published in Philadelphia in 1853. It makes a very thorough pro-slavery argument, which is mostly grounded in racism/classism/sexism, but its most convincing pieces are its treatment of slavery in the Bible. It points out that slavery under the Romans was often as harsh as under Southerners, involving working slaves to death in mines. It points out that both the Old Testament and the New Testament affirm slavery, in the OT specifically the chattel slavery of non-Jews (Lev. 25 44-46). Paul writes a whole epistle, that of Philemon, urging a slaver owner to take back his house slave and treat him well. Not free him mind you, but just treat him nicely. It only once means the Hamitic theory, to call black people the "posterity of Ham" in the midst of another argument.
On the merits, the abolitionists had a much weaker Biblical argument than the slavery advocates.
I'm not an expert in this field, and there are already a number of great answers, so the mods may wish to delete my comment. But a couple of years ago I did a little bit of study into the racist literature and propaganda of the antebellum South. One fascinating work I came across was Aunt Phillis's Cabin, published in 1852 as a response to Uncle Tom's Cabin. If you browse through the book's preface you will see how Christianity and the Bible were used to justify slavery. It's a truly staggering (and frightening) account.
*AUNT PHILLIS'S CABIN; OR, SOUTHERN LIFE AS IT IS.
It's a long book, but I would encourage people to at least skim through parts of it to get an impression (as if anyone needed it) of how white Southerns felt about slavery and Northern "meddling" into their "sacred rights." According to Wikipedia, "the novel sold 20,000-30,000 copies, making it a strong commercial success and bestseller."
I apologize if this comment is not up to par. I just thought that I may add a bit to the conversation.
I do not have my books with me at the moment, but a monograph titled, Fatal Self-Deception, by Eugene and Elizabeth Fox-Genovese dedicates a good deal of itself to the justification of slavery, mostly by white southern planters, vis a vis biblical scripture. One of the main thrusts of the work is that many southern planters truly believed that it was not only their right, but also their religious duty to hold slaves. They believed that as white men it was their place to act as paternal figures to blacks, or any other minority, for that matter. Generally speaking, white planters assured themselves that blacks would die out as a 'species' (forgive me, I am almost certain that in the book it is discussed that many planters, at least initially, viewed blacks as a separate race. I may, however, be wrong.) Thus, planters saw themselves as unappreciated heroes, rather than reprehensible villains. As time wore on sentiments such as the above did give way to more progressive views, but some still fought tooth and nail against believing that slaves were worthy and able to be free. If you have the time, give it a read. It utilizes a great deal of primary and secondary sources from both sides of the aisle, as well as both sides of the field. It does get a bit drab at some points, as the source material tends to give more than a handful of examples for each one of the points it attempts to make, but the book is, nonetheless, a very welcomed change of pace.
If there is anything I can mention, please let me know and I'll do my best to recall.
Fatal Self-Deception: Slaveholding Paternalism in the Old South ISBN:9781139153041
This is also an issue in Islam. Last week I had a discussion with a Muslim Emam from Mauritania and he said that slavery is still too commonplace there. He defends the practice and state that its mentioned in Quran and Hadith