How was slaveholding seen as a "perversion of Christianity" in Narrative of the Life of Frederick Douglass?
The answer to your question is indeed all over the place in Frederick Douglass' Narrative -- from the comments in the preface to the parody of church music at the end -- and while I agree completely with akannan that you will need to do the legwork to identify the quotations, I want to share a legitimate strategy that you may not yet know about or use: the full text of this slave narrative is available online (see the two links below for sample sites housing the text).
Electronic texts are searchable, which means that you after you access the electronic text you can use the "Find on this page" command (under "Edit" in the menu of Internet Explorer) or simply the keyboard command "CTRL-f" to open up the search function. Type in one keyword at a time -- such as "Christ" (which will probably also find "Christian") and "religion" -- and you'll find every occurance of each word in the electronic text.
You will still need to locate page numbers in order to put the quotations in MLA format, and page numbers are not often given in electronic texts. The chapters in Douglass' text are short, so you may find yourself reviewing your print copy of the book (by chapter) to find out the exact page number for the relevant quotations in the chapters that you have searched electronically.
Selective searching of electronic texts is nowhere close to the same thing as reading the full text, of course. I recommend using this strategy after you have already read the text in full one time through but don't necessarily want to read the entire text again just to find relevant quotations. Douglass' story is amazingly good, and if you simply search instead of read, you're really not gaining anything through the assignment.
I think that you are going to have do much of the legwork on being able to identify quotations from the text that help to bring out the contradictions with Christianity. There is much in the work to bring out this "perversion." It represents a critical theme of Douglass' work and life as a social activist. Any nation or individual that believes in Christianity cannot reconcile this belief system with the reality of slavery. Douglass goes to many ends in his writing to point out the hypocrisy involved in Americans that practice both Christianity and slavery. I would pay special attention to how Douglass describes slaveowning in the South, individuals who possessed a strong sense of faith, but actively believed in slavery. Douglass uses them as examples of this hypocrisy.
You should also go directly to the Bible to see what it teaches about slavery. It is not condoned at all. In Biblical times, slavery already existed, so when the law was given to Moses, it referenced slavery. In ancient Israel, slavery was more like indentured servanthood, or "voluntary slavery" as it is sometimes called. God also provided a way of escape for slaves - every seven years (read Deuteronomy). Basically, involuntary slavery is not Biblical and is a perversion of Christianity, where we are told in Paul's letters that we are all the same in Christ Jesus - "There is neither Jew nor Greek, slave nor free, male nor female, for you are all one in Christ Jesus" - Galatians 3:28. For an excellent discussion of this, see the link below.
In Fredrick Douglas' book, he identifies that the men who are slave holders speak the Word of the Christine doctrine but don't practice it. They actually use Christianity to validate their ownership or slaves and slavery. At one of his presentations he tells of what Christian beliefs are which are contradictory of the way in which slaves are treated. He finds it ironic that the faith calls for mercy but slaves are not shown mercy.
It is the peculiar characteristic of Christianity that it is a code of mercy,—that it interests itself in the welare of man,—and is ever ready to lend its ear to the distressed, and to send them succour.
Douglas had presented the details of ownership the night before in a speech in which he exposed the cruelties and denial of rights experienced by slaves. He used these situations to reflect light on how the opposite behavior of what Christianity is meant to be is played out daily in the life of a slave by Christian owners.
The slave holders resort to all kinds of cruelty. If I were disposed, I have matter enough to interest you on this question for five or six evenings, but I will not dwell at length upon these cruelties. Suffice it to say, that all the peculiar modes of torture that were resorted to in the West India Islands, are resorted to, I believe, even more frequently, in the United States of America. Starvation, the bloody whip, the chain, the gag, the thumb-screw, cat-hauling, the cat-o'-nine-tails, the dungeon, the bloodhound, are all in requisition to keep the slave in his condition as a slave in the United States.