I found a couple of answers on this topic, but I wanted to know if religion was used as the main argument or a secondary, not so relevant one, to keep slavery going.
Religion was used by both sides of the debate. One letter I remember in particular reading a few years back was written by a slave to the arch bishop of Canterbury. The writer stressed that he and his fellow slaves had been going to church and were becoming good christians, and that the bible said it was not OK to enslave other christians. He therefore argued that he and his fellows should be freed.
It was a heartbreaking letter, and really reads as if the author is convinced that if the archbishop was aware of what was going on in America (still colonies at the time) he would put an end to it. It was a genuine heartfelt plea for help, and the writer stressed that if his master found the letter, he'd surely be killed.
Important to note also is that religion was much more a central part of life in the New England colonies early on. Virginia and many of the southern plantations were very profit-oriented, seeking to make as much money as possible in any way shape or form. Plantation owners used the bible to justify their actions and to help the formation of laws regarding slavery (I recall reading that several women in Virginia were arrested for bringing their slaves to church with them), but it's unlikely that they truly believed it. Conditions in the tobacco plantations were so bad that even the most desperate European peasants soon refused to make the commitment of indentured servitude. The plantation owners turned to slavery to fill their labor needs, and as no system of law within the English system really allowed for the institution of slavery, laws needed to be created, and they were eager to justify it in any way they could.
Frederick Douglass is a great source, if you have never before read narrative.
"The church and the slave prison stand next to each other. ... [T]he church-going bell and the auctioneer's bell chime in with each other; the pulpit and the auctioneer's block stand in the same neighborhood."
http://www.ibiblio.org/ebooks/Douglass/Narrative/Douglass_Narrative.pdf
I've been trying to search on the library of congress' site through the old congressional globe records. It's difficult to search through, however, because pdf... At my law library, we have a physical set of the congressional globe that's much easier to look through. Anytime slavery is defended on the house or senate floor, the justification almost always includes god, or christianity, or the southern way of life that god ordained. You can try to search the loc yourself.
Lots of good answers here, just wanted to add that the specific passages cited by most slave owners, especially when preaching to their slaves were Colossians 3:22 and Ephesians 6:5-9.
Source: Eric Foner's Reconstruction
While not expressly supporting slavery, the curse of Ham was used as a Biblical backing for enslavement and the dehumanization of black people. I.e. it never says anywhere in the Bible that black skin was a result of the curse, but it was a popular myth throughout Europe and America and was used in support of slavery. However, there were direct connotations of the curse causing dark skin in some Jewish and Islamic texts, which may have influenced Christian beliefs.
On top of that, there is also the curse of Cain which was used in a similar manner as well as LDS teachings that black people were fallen angels.
I've only studied this for 18th Century Massachusetts (which kind of breaks your question a little since it was a Colony for the most part), and in the discourse of the period, The Bible was kind of the one-stop semiotic shop for all arguments. If you want to flex your reading muscles, this is an excellent primary source for the counterargument though Saffin's argument, which centers on Ham, Canaan, the enslavement of peoples by the Israelites and so on, is much harder to find though it would answer your question more directly. The Saffin, if you can find it, is wonderful because the arguments used in it were used again and again, later in the 19th century arguments for slavery and even later still in the 20th century for justifying segregation. It is really only since the civil rights era that arguments about the Curse of Ham and the roles for Noah's heirs have left public discourse.
Really my point with this is that the Bible was used on both sides of every argument, and really understanding the discourses of the American past is really helped by having a basic grasp of Christian theology.
Religious arguments were used on both sides of the slavery debate, both by slaveowners and by abolitionists. It seems doubtful, however, that Scriptural arguments played a fundamental role in shaping people's opinions of slavery; it seems more that the Bible was used by both sides to justify whatever moral position they already held. In this 1860 sermon by Rev. Benjamin M. Palmer (of New Orleans), several religious allusions are made to justify slavery, and yet the argument itself is not scriptural:
[W]hat, at this juncture, is [our] providential trust? I answer that it is to conserve and to perpetuate the institution of domestic slavery as now existing. . . . Need I pause to show how this system is interwoven with our entire social fabric . . . and sanctioned in the Scriptures of God . . . ? Must I pause to show how it has fashioned our modes of life, and determined all our habits of thought and feeling, and moulded the very type of our civilization? How then can the hand of violence be laid upon it without involving our existence?
[qtd. in Goldfield, et. al., The American Journey: Brief Edition Combined Volume (6th Edition), pg. 389.]
T.R.R. Cobb was the brother of Georgia governor Howell Cobb, and was a prominent lawyer. He helped found UGA's law school and even wrote Georgia's legal code! He did fighting for the Confederacy in the Civil War. Diehard secessionist, so he represents how the slaveholding Southerners justified themselves.
One of his proudest accomplishments was writing An Inquiry into the Law of Negro Slavery in the United States, which uses the law to justify slavery but also traces slavery back to Noah's time. " From the familiarity with which Noah spoke of the servile condition of his youngest son, it seems probable that the condition of servitude must have existed prior to the flood." It's appallingly racist yet also well-researched, somehow?! Give it a quick look before you get too angry.
Recently New Books In History interviewed Luke Harlow about his new book which covers this issue in Kentucky. B/c Kentucky was a border state there was pretty much every side of the issue in conflict and those conflicts developed more quickly b/c of proximity. It was more than a pro/con argument with shades of gradualism, deportation, and limited rights arguments. It was an interesting interview. Here's a link: http://newbooksinhistory.com/2014/06/26/luke-e-harlow-religion-race-and-the-making-of-confederate-kentucky-1830-1880-cambridge-up-2014/
An interesting book (or at least chapter in the book) on this topic is "Eve and Adam: Jewish, Christian, and Muslim Readings on Genesis and Gender" - Indiana U Press (1999). Chapter 7 discusses some of the religious arguments used in defense of slavery and also those arguments used to argue for abolition. The reason slavery is mentioned in a book about gender is because many of the same religious arguments that women's rights advocates were using were also used by abolitionists. If I remember correctly there's also a supplemental chapter at the end of the book that discusses groups that still exist that use religion as a justification for race segregation.
Anyone know what happened to elderly slaves who could no longer work? Or the sick ones? I'm guessing calling in sick was not an option.