This is actually a big thing that a skeptic has to deal with, especially for our own beliefs. We all have a natural inclination for confirmation bias. We quickly reject that which we don't agree with and quickly support anything that matches our position. The beauty of the human mind.
There's also an element of wanting to defend our beliefs. There are plenty of cases where professors and very intelligent people remain stubborn just because they don't want to admit that they were wrong. If you enter a debate with the intent of defending a possession, you usually just look for evidence that supports you. I've noticed myself doing this at times. You have to actively watch your own train of thought to sort this out.
In the same way, arguing against someone usually just encourages them to dig in harder. For the religious (especially for Christians) there seems to be a real desire to play the victim. Religions in general thrive and form a strong group of core believers when faced with challengers.
That said, when I generally argue against anything (crazy political theory, conspiracy theory, religious theory, etc.) I'm not doing it for the person I'm debating with. They're usually too far gone. They will only come around when they feel like it or when their mind snaps back into place after a good discussion.
Debates, especially those on forums, are usually for those on the margin. Skeptics win by showing the holes in religious arguments and presenting the other side to onlookers who really do want to know both sides of the story.
Just my 2 cents though.
