The first thing you need to learn is that facts don't matter to people as much as you think. Someone can't be reasoned out of a belief if they've never learned how to reason in the first place, and so trying to hammer a person with facts and logic accomplishes the exact opposite of what we want to achieve. Study after study confirms that as people are presented with contradictory evidence to their beliefs, their beliefs actually get stronger.
So if facts don't matter, how can we reason with people? Well, it turns out that 'reasoning' is something people have to do for themselves. You'll find that most religious people have never fully reasoned out their beliefs. What percentage of Christians have actually read the entire Bible? Again, it isn't helpful if you're the one to point out the unpleasant elements of the Bible to them. This must be a process they discern for themselves. All you can do is encourage the process by asking pertinent and pressing questions about their beliefs. Remember, these are questions; not answers.
Of course, who wants to wait for that to happen on its own? You'll find that the best way to get a person to feel ignorant is to get them to try and 'teach' you what they believe. The key is to have the more profound understanding of the material than they do. This is where facts do matter. Knowledge of the Bible, for instance, allows you to probe deep enough into a person's belief to challenge them to learn more for themselves. If a person is religious, it is usually because they are deeply ignorant about all the facets of their beliefs. Most people really don't think about their religion much; but when uncomfortable or even outrageous things about their religion bites them in the ass, it's an experience that's difficult to sweep under the rug.
It reminds me of a story I saw on the news a few years ago. It was about a preacher who had decided to focus his sermon on some of the more sexist elements of the New Testament (like Thessalonians, telling women to be silent). A parishioner was then interviewed, and she expressed outrage over it. She just couldn't believe that this would be in the Bible. Talk about just a tame example from what is arguably a pretty gruesome tome.
It's these kinds of moments that begin to help an individual unravel the tapestry of their own beliefs. Once their schema begins to change, only then will facts seem to matter. At that point, they may even have a voracious appetite for them! —Jacob Fortin