I often hear the question, and have asked the question myself, why do people in this day and age still believe in the bible? (of course the Qur'an could fit in here too)
The thing to keep in mind is that people wrote and believed in the bible when they didn't know better. They not only didn't know how to properly investigate things, they didn't know they could even try. People used to believe the wrath of god was an actual thing they witnessed. It was a time and place when people believed all kinds of nonsensical stories and there were no scientists around to say 'let's see if we can disprove that'. Claims were just accepted without question. (still the case for many people today)
Add to this the recognition (eventually) by the ruling classes that religion was a great method to control the masses and suddenly you've got a very powerful tool behind you.
So with no way to investigate claims and authority figures reinforcing the myths at every opportunity it really is no surprise that fantastical stories were believed by entire societies.
Given that we don't have this excuse now, why do people believe today? Think about it... In modern times when do people become religious? Is it when they've reached the age of reason? In their late 20s when they've had a solid scientific education first? Never (or extremely rarely if ever). In actuality, for the vast majority of cases, religion is driven into people when they're infants. It happens when they're children, it comes from a trusted source, and it's forced upon them without their consent. They grow up 'knowing' it to be true because they're never given a choice. They then think they're doing a good thing when they continue the cycle onto their own children. It takes a spark to break the cycle but often that spark never comes so the cycle of belief continues.
I've read about people going off to university and their path to atheism beginning there. For many it's the first time they meet and interact with people of differing views. It's the first time they learn that not everyone believes what they do, that people doubt the validity of religious beliefs and they do so with good reason. This is the 'spark' they need to start questioning what they believe.
But many people are surrounded by believers their whole lives. The spark to make them question what they've believed from such a young age and with such totality never occurs and as a consequence the path to atheism is never encountered.
I think that's why the internet is so important, it helps get information to people that would otherwise never be exposed to it. It helps show people there are differing views and that questioning your own beliefs isn't necessarily a bad thing. I'm not suggesting that a single tweet or even a series of them can turn a believer into an atheist but they can be the spark. It's the 'hang on a minute' moment. People then ask more questions, read more books (books that aren't part of their scripture), do their own investigation and from THIS work they conclude that atheism is the logical position for them.
I've been very fortunate to have quite a number of people tell me I've either helped them on their path to atheism or helped show people who were already atheists that they're not alone. I see this as probably the best part of being a vocal atheist. The key, as I see it, is to answer questions openly and honestly - as long as those questions are genuine. Of course sometimes people just want to troll and waste time and how you deal with them is up to you. My opinion - best ignored.
So I recommend engaging with theists. Talk to them. On The Atheist Experience they ask a great two part question to get the conversation going 'What do you believe and why?' I recommend it.
People believe because they've never known anything different, because they believed since before they can remember and they were never given the choice. People believe because they were never given the spark to ignite their doubt, the spark to ignite questions.
Be the spark.
