Considering atheism [Currently Christian]

[Astonished]

Why is it irrelevant if science is unable to test or verify it?

That which cannot be verified cannot be usefully discussed as it hasn't exited the realm of hearsay and speculation. It's not irrelevant, as it might be worth pursuing further study of to gain more facts, but in the meantime, it's not actionable, either. For example, we don't know what dark matter is or even if it exists for sure. It's a hypothesis at this point, we have some evidence pointing to it, some against. It's not irrelevant, in fact it's a subject of keen interest, but we are reduced to speculation (educated or otherwise) until we know more.

Dark matter is an example where you at least have a falsifiable, scientifically valid hypothesis to work with and test. Invisible beings and realms / the spiritual / the supernatural are all unfalsifiable concepts that no one can make credible knowledge claims for or against. They are just asserted without evidence, and so are dismissible without evidence.

I should have clarified, that primarily applies to those things which people define as specifically being 'forever' outside the reach of science. You know, the pathetic dodge to ensure that science can never disprove god even if a means to do so someday presents itself, depending on definitions. Gotta love religious wordplay, so their tap-dance becomes a sing-along too.