They have an emotional attachment to their beliefs. To change their beliefs would be emotionally painful for them. It is the same with anyone with a belief they are unwilling to review -- you don't need religion to find it painful to review a belief you are emotionally attached to -- scientists suffer from this as well. (Imagine if you had invested an entire career in a mistaken theory!) If you really want to change their mind, you need overwhelming evidence, and expect them to have to go through a kind of emotional breakdown and a week or so of uncertainty until they are able to rebuild some kind of world-view which works with the new evidence.

So, probably you won't persuade the most vocal posters, but the lurkers are still reading your words.