(I Can't Get No) Satisfaction

The scene is a tangled bank, some time between 1848 (when Cecil Frances Alexander's hymn All Things Bright and Beautiful was first published in Hymns for Little Children) and the publication of Darwin's On the Origin of Species in 1859.

Enter a BELIEVER and an UNBELIEVER, deep in conversation, as they contemplate endless forms most beautiful and most wonderful:

BELIEVER

Do you not see the Creator's handiwork all around? See the irises, the dragonflies and the kingfisher. As Mrs. Alexander so charmingly puts it , 'He made their glowing colours, He made their tiny wings.' Truly, the Lord God must have made them all!

UNBELIEVER

I beg to differ.

BELIEVER

How, then, do you explain the wonderful artifice and intricacy we observe in even the humblest creature?

 UNBELIEVER

I confess that, although I find your explanation fantastical, I do not know. It is a mystery.

 BELIEVER

I offer an explanation, but you can say nothing.  I have the better of this argument, I fancy.

Although unbelief predates Darwin by many centuries, I've always been impressed by Richard Dawkins' argument (in The Blind Watchmaker) that it must have been very hard to be an intellectually satisfied atheist before Darwin. 'God did it' might not be a wholly satisfying explanation for apparent design and complexity in nature, but it must have sounded a lot more plausible when the best nonbelievers could come back with was 'I don't believe that God made complex things, but I've got no idea why they exist.'

A century and a half later, the boot's on the other foot. Unbelievers have an explanation that's far more robust, plausible and rigourously tested than the theistic 'the thing that made the things for which there is no known maker and that causes and directs the events we can't otherwise explain and doesn't need to have been made, made them all.'

More thoughtful and intelligent believers are now the ones on the back foot. Instead of challenging unbelievers with anything as compelling as the analogy of the watch from Paley's Natural Theology,* they're now reduced to arguing that, although they can't provide an intellectually satisfying argument for the existence of God, God's existence is just not the sort of thing you can conclusively disprove.

Scriptural literalists and creationists who'd like to challenge unbelievers more forcibly are hampered by being completely, and demonstrably, wrong.

The best lack all conviction:

I can't prove it. I don't know that any of it is true. I don't know if there's a God. (And neither do you, and neither does Professor Dawkins, and neither does anybody. It isn't the kind of thing you can know. It isn't a knowable item.)

...while the worst Are full of passionate intensity

*If you're looking for a better argument for the existence of God than 'well you can't conclusively prove He doesn't exist,' I suppose there's still the argument from fine tuning, but it sounds like clutching at straws to me.