I was asked by a self-described "traditional Catholic" why faith is a bad thing. It's a good question that deserves an answer, and here's mine. Since so many of us are most familiar with it, let's examine Christianity. Over two billion people currently have faith that Christianity is the One True Religion/Belief/Faith/Whatever. Millions of Christians believe that Jesus is their god and savior, while millions of others just believe that Jesus is just their savior but not god while still others just believe that Jesus was a good man with a good message that they try to follow.
Do you see where I'm going with this?
Billions of Christians believe that Heaven and angels are real. Many of them believe that Hell is also real, but not all. Many of the people who believe in Hell believe that anyone who have not explicitly sought salvation go to Hell, while others claim that all good people go to Heaven and only the truly wicked go to Hell. Still others believe that no one goes to Hell, it's only a place for Satan and his demons. Some believe that Hell isn't real at all, that it's a misinterpretation or mistranslation or outright forgery in the Bible.
These are just a handful of disagreements over the dogma of the largest religion in the world. Whoever is right -- if any of them -- can have profound implications for belief and humanity as a whole. Every one who believes whatever variation of dogma has a strong foundation for what they believe and why they believe it. They can quote scripture, cite authorities and argue endlessly about why their belief is the correct one. The only thing they have in common is faith. They all have faith that their beliefs are correct, even though their beliefs can't possibly be all correct.
Yes, faith does interfere with reason. In an argument with me over my atheism a family member declared that I have to abandon what I know to embrace faith, because faith is superior to knowledge. If my eyes observe something that contradicts my faith, I should reject what my eyes see and embrace faith. Ken Ham echoed this in his debate with Bill Nye last year when both were asked what would change their minds, if anything. Ham said "nothing." Faith is his bedrock and he will not be moved, no matter what facts may contradict it.
Faith does not bring us closer to the truth. When we use the scientific method to explore a question, understanding converges. When we explore a question through faith, understanding diverges. The end result is that we add confusion to already complex topics and hinder our efforts to arrive at real, practical answers.
So I have to go with Mark Twain on this one: faith is believing what you know ain't so.