What makes people irrational thinkers?

[polymath257]

The question which then follows; is why should anyone?

If there is no metaphysical methods that can reliably discern fact from fiction, wouldn't that mean, that those that believe in said god claims, are being irrational?

According to your own metaphysical beliefs, only science can discern fact from fiction. 

If it's important to you that others accept your metaphysical beliefs, then of course you should use philosophical arguments to persuade them.

Well, many have a rather limited concept of what constitutes 'science'. I would label any subject that uses the scientific method as a science.

In particular, there is no predefined notion of 'material' or 'physical'. All that is required is that hypotheses be testable by observation.

Now, there are subjects not subject to this method. For example, mathematics is not based on observation, but is instead a formal theory based on axioms and deduction from those axioms. It has a different test for the reliability of its ideas. But that makes it correspondingly limited in what it can say about the 'real world'.

I am more than willing to accept that other subjects may have other methods for eliminating falsehoods. But the two methods I know that work (hypothesis testing by observation, and proof in a formal system) don't seem to work in metaphysics. And, as you have admitted, there is no accepted way to eliminate falsehoods from metaphysics. That lack means that is it a subject of opinion and not of knowledge. It is closer to aesthetics than it is to science: a matter of taste and not a matter of objective fact.