The problem is when they start voting based on those beliefs and help elect representatives who then attempt to make policy for all of us based on those beliefs.
Lastly when you realize not to worry about people with illogical beliefs, and instead focus on educating those who are either open to information, or already support your beliefs you'll focus your energy on the things that you can actually change. Yelling, worrying or typing "you're wrong" to dogmatic religious adherents, homeopathic healers or conspiracy theorists is not going to change their mind.