Why is it important to convince a theist that he is wrong?

(quick rant)

People allow, nay, insist that their religion inform their opinions on social, civil and political matters. This necessarily has an effect on everybody.

Even if someone's religion happened to point them in directions beneficial to mankind, their mere ability to hold a belief which is based on groundless hearsay and that horrible F word, indicates that they can not be trusted to use sound means to deliberately inform their opinions in AT LEAST one matter.

If someone can believe something crazy without harming the rest of us, no problem. But that's not how it works. If someone wants to demonstrate that having reviewed a great breadth and depth of "evidence" they've come to the conclusion that there is no reliable reason to believe... but they still believed... as long as they recognize that their supernatural beliefs have no place in deciding how best we may live, work and advance together, there's no problem. But that's not how it works.

Why is it important to convince a theist that he is wrong? Because it hurts us all, in small but cumulative, massively cumulative ways.

Can theists be convinced by reason?

They can be reached by reason. However not all avenues are assailable. That which may be believed without reason may be immune to reason. The only hope is to discover the underlying foundation to beliefs and addressing those.

Belief is a subjective response to the best (also subjective) available evidence. Misinformation can be more valuable in making a decision in the mind of the subject. A convincing and engaging liar may appear more credible than a fact laden empiricist. We can try to teach and motivate our acquaintances to be more demanding in their epistemology. But convincing people to think harder and to consider the counter-intuitive is exhausting and poorly rewarded.

Generalized reasonable atheism advocacy will find purchase in some individuals. But we cannot predict whether such advances remain chronic moot doubts, get rationalized away or lead to serious inquiry. We probably won't hear about many near-success stories. Here too, general refutation of theologies and super-naturalism is exhausting and poorly rewarded. But retreating from either front would be a tacit invitation to every charlatan and would-be prophet.

When dealing with fallacious beliefs on a case-by-case basis we may ask the individual what they believe and why they believe it... and why they believe that... and why they believe that, until you either reach a point of mutual agreement or it is demonstrated that there is no reasonable foundation to the larger belief matrix. If you are rigorously skeptical and don't fall for any false syllogisms, reason may prevail.

Most religious ideologies encourage adherents to share their beliefs so it shouldn't be that hard to open this dialog. But questioning the beliefs is generally not welcome so expect that to get sticky. Whether I believe that milk is good for stronger teeth and bones or that dreams come from ethereal creatures, if I'm not willing explain why I believe these things I should not be treated as credible. Beliefs may be completely out of touch or demonstrably factual. It is the responsibility of the proponent of any belief to explain why it is true. Responsibility.

In the end every believer is left with "evidence" and reasons that are not substantially better than those for most competing supernatural beliefs. All the believer has left is faith and subjectivity. Not coincidentally, believers of most competing supernatural beliefs are identically "supported" by their faith and subjectivity.

They can be reached by reason. But your best bet is to have them search for reason, then let them wonder why they can't find it.