I don't advocate arguing about religion with fanatical Christians. They have God on their side, after all, and all you're armed with is the paper swords and cardboard shields of Satan's deceptions. If you do argue with a Christian enthusiast, expect to be told something like that.
However, if the debate strays from faith and into more concrete topics, such as American history, you'll have a chance. It's likely your opponent will come at you with bogus quotes to "prove" that America is a "Christian nation." These are easily refuted with a little research.
If you do find yourself pressed by a Christian literalist in argument, you might want to turn the fight as quickly as possible to their ground -- the Bible. Once you're on the Bible, you can pick at your leisure from any one of the dozens of possibilities: Internal contradictions, the ridiculous cosmology (flat earth, etc.), the cruel pettiness of the O.T. god.
If you're going to often find yourself in such debates, best if you can learn enough classical Greek to be able to read the thing in the language the New Testament was written in. It's an eye-opening experience, for one; and it lets you cut through a lot of crap that got in there by bad translation. It also gives you the ultimate trump card, if you know what it says and your opponent doesn't. And your opponent probably won't.
An astonishing number of people who claim the Bible as the touchstone of their lives, the one book they need to get through the day, the instruction manual that they would impose on you and me and everyone else -- CAN'T READ IT. Reading a translation isn't the same thing. Not even close. With the Bible, you've got speeches that Jesus made to crowds of people in a long-dead vernacular Hebrew called Aramaic, written down years later in classical Greek with varying degrees of competence (Luke's Greek is elegant; Mark's is wretched), then translated into English sometime between 1600 and now (depending on which English version of it your opponent always carries with him).
I wonder why some of the people so obsessed with God's Word don't try to meet him halfway on it. It's difficult, but not impossible, to learn ancient Greek. It's also worth it if you plan to be any sort of atheist or non-Christian in America.
If you must debate Christians, be alert for some of their favorite logical fallacies. The idea of a logical fallacy is neither Christian nor pagan nor secular. It's something like a wrong turn in the map of thinking.
A common example, for instance, is the fallacy of accident - confusing what is accidental with what is essential. In Latin, this was described as cum hoc ergo propter hoc ("with this, therefore because of this"), or, in a slight variation, post hoc ergo propter hoc ("after this, therefore because of this"). Both these names describe the mistake. This is the fallacy of assuming that A caused B simply because A happened along with B, or prior to B.
Here's what the fallacy looks like: "Most rapists read pornography when they were teenagers; obviously, pornography causes violence toward women." But the two phenomena can have a correlation without one causing the other. It is possible, for instance, that some psychological factor might cause both a tendency toward sexual violence and a desire for pornographic material.
Or it might not. Assuming that something is false simply because a proof or argument that has been put forth for it is invalid is itself a logical fallacy (argumentum ad logicam, if you're keeping score in Latin -- the "appeal to logic"). There may be another proof or argument that succeeds.
Argumentum ad logicam matters because it's a favorite of Christians, under the guise of the "straw man argument." This is the fallacy of refuting a caricatured or extreme version of somebody's argument, rather than the actual argument they've made. The metaphor is of someone who builds a scarecrow, knocks it down, then gloats. "Boo-yah! How you like me now!" Creationists are fond of saying that scientists think that complex life "just happened" or "fell together at random." That's a straw man argument.
Another favorite fallacy is Petitio principii - "begging the question," pretty much indistinguishable from circulus in probando, or circulus in demonstrando "arguing in a circle;" demonstrating a conclusion by means of premises which presuppose that conclusion.
This fallacy may inhere in a single word or phrase ("Old Testament"). Or it may look like this, my favorite example, from a child's conversation: "I'm glad I don't like brussel sprouts." Why? "Because if I liked them, I'd eat them. And I hate them."
It's used in political debates, often with little more disguise than this: "Marijuana is illegal. And we all know that you shouldn't violate the law. Since smoking pot is illegal, you shouldn't smoke pot. And since you shouldn't smoke pot, it is the duty of the government to stop people from smoking it, which is why marijuana is illegal."
The Christian version is drearily familiar: "God is real." How do you know? "Because the Bible says so." How do you know the Bible is correct? "Because it was inspired by God."
You'll also likely encounter the fallacy of non sequitur - ("fallacy of false cause"), a conclusion based on insufficient or erroneous reason: "Statistics show juvenile delinquency is rising. Therefore, we need to post the Ten Commandments in public schools."
Look out for the fallacy known as special pleading. - "God moves in mysterious ways." "The lord reveals knowledge to the ignorant that he hides from the wise."
Another Christian favorite is the excluded middle/false dichotomy fallacy, which reduces complex situations to only two possibilities -- "Love it or leave it;" "you're either part of the solution or part of the problem;" "who is not with me is against me" -- this last from Jesus himself, showing just how deeply it's engrained in Christian thinking.
Christians love dichotomy; it's rooted deep in their faith. Good-bad, Christ-Satan, heaven-hell. Alcoholics Anonymous confronts people with this dilemma: "To be doomed to an alcoholic death or to live on a spiritual basis are not always easy alternatives to face .... But after a while we had to face the fact that we must find a spiritual basis of life -- or else."
This fallacy is often masked as an appeal to avoid the "slippery slope." A variant is reductio ad absurdum, where one carries the opponent's position to its logical end, without mentioning that this is not the inevitable result of the opponent's viewpoint. How often have you heard this one: "If we legalize marijuana, the next thing you know we'll legalize heroin, LSD, and crack cocaine."
One way some Christians use this is to say, "Atheists have no source for their morality; therefore, they have nothing to stop them from committing rape and murder." This ignores the fact that most atheists think long and hard about right and wrong and do have sources for their morality: Reason and an inherent sense of justice that lets them reject even the Judeo-Christian god when he tells his followers to rape and murder.
Many fallacies can be grouped under the heading "irrelevant conclusion," where instead of proving the facts in question, the arguer seeks to divert attention to some extraneous fact. Some forms this takes are:
argumentum ad hominem - "argument from personal considerations;" attacking the character or motives of a person who has stated an idea, rather than the idea itself. For a flood of examples, read any Christian literature on Madalyn Murray-O'Hair, John Dewey, or anyone else they don't like. They may have been genuinely ugly people. But that doesn't make their ideas or their causes wrong. Any more than the Inquisition's excesses invalidate all of Christianity.
argumentum ad populum - "appeal to popular sentiment;" and the nearly identical argumentum ad numeram - appeal to numbers. This amounts to attempting to prove something by showing how many people think that it's true. "According to a recent Gallup poll, 68 percent of Americans favor teaching creationism in public schools." Well, maybe 68% of Americans are stump-ignorant about science, education, and the Constitution. Ad populum is construed narrowly to designate an appeal to the opinions of people in the immediate vicinity, perhaps in hope of getting others to jump on the bandwagon, whereas ad numeram is used to designate appeals based purely on the number of people who hold a particular belief.
argumentum ad ignorantium - "appeal to ignorance;" whatever has not been proved false must be true, and vice versa. Another creationist favorite.
argumentum ad baculum - "argument to fear," and the related "argument from adverse consequences": "The court must impose the death penalty in this sensational case as a deterrent." For many people, this is the whole crux of the Christian faith: "Do what God tells me to tell you to do, or else you'll roast in Hell for all eternity." Except for the Calvinists, who say "Do whatever you damn well please, you're probably going to roast in Hell for all eternity anyhow and there's nothing you can do about it."
Also, look out for the naturalistic fallacy. This is the fallacy of trying to derive conclusions about what is right or good (that is, about values) from statements of fact alone. No matter how many statements of fact you assemble, any logical inference from them will be another statement of fact, not a statement of value. If you wish to reach conclusions about values, then you must include among your assumptions (or axioms, or premises) a statement of value. Once you have an axiomatic statement of value, then you may use it in conjunction with statements of fact to reach value-laden conclusions.
Christians of a certain sort have a natural tendency to mistake their morality for the only morality, and their values for the facts of the universe.
Also, look out for these:
fallacy of many questions (Plurium Interrogationum) - several questions improperly grouped in the form of one, for which a direct categorical answer is demanded. Similar to fallacy of complex question, a favorite of a former girlfriend of mine, which implicitly assumes something to be true by its construction ("Why do you hate me?"), or the classic, "Have you stopped beating your wife?"
observational selection - counting the hits and forgetting the misses. A favorite of "Biblical prophecy" fans.
statistics of small numbers - Carl Sagan summed it up something like this: "They say that one in five people in the world today is Chinese, well, I live in Indiana and I must know thousands of people and not one of them is Chinese."
argumentum ad verecundiam - "argument from conventional propriety." Closely related to "argument from authority:" citing some person who agrees with you, even though that person may have no expertise in the given area -- Einstein on politics, for instance. This is also similar to argumentum ad antiquitatem -- "Every great civilization in history has provided state subsidies for art and culture." But that fact does not justify continuing the policy.
argumentum ad nauseam - "argument to the point of disgust," especially by repitition. Trying to prove something by saying it again and again.
Fallacy of the consequent - arguing from a consequent to its condition: "drug addicts live in slums; therefore people who live in slums are drug addicts."
Secundum Quid - arguing erroneously from a general rule to a particular case without regarding special circumstances which vitiate the general rule; or the converse fallacy of arguing from a special case to a general rule.
This example is also a case of the fallacy known as dicto simpliciter, literally "spoken simply," the "sweeping generalization" that is presumed to be true of every specific case -- in other words, stereotyping. Example: "Women are on average not as strong as men and less able to carry a gun. Therefore women can't pull their weight in a military unit."
But, like I said at the top, I really think arguing with committed Christians about anything is a bad idea.
"The virtue of using evidence is precisely that we can come to an agreement about it. But if you listen to two people who are arguing about something, and they each of them have passionate faith that they're right, but they believe different things -- they belong to different religions, different faiths, there is nothing they can do to settle their disagreement short of shooting each other, which is what they very often actually do."