What would it take to convince you of Creationism (that is, a god or gods created the universe)?
Do you mean "Creationism" as in God just kind of formed everything the way it is now pretty much instantaneously a few thousand years ago? It would be virtually impossible to convince me of that short of disproving all of modern science (chemistry, physics, geology, biology, and so forth) and then building an adequate case for it.
Or do you mean convince me that God created the universe through the Big Bang or some other method that is consistent with rational thinking and the evidence we have? I'll answer for the rest of my post as though you meant this option, as this option is worth discussion in my opinion.
No, too subjective. A snowflake looks designed but it forms from rather basic laws.
No, not convincing enough. If, let's say, our fine-tuned constants have a 1 in 10^30 chance of happening that way, but we exist, then they happened that way. Add in the concept that a possibly infinite number of universes could exist, and fine-tuning becomes somewhat irrelevant in my opinion. If you have 10^30 universes and 1 in 10^30 randomly are tuned for life, then the one universe that has life will see their universe as "fine-tuned".
I'd love to see how extra terrestrials view religion and cosmology. I wouldn't just take their word on anything, though. If they were advanced enough in technology to come visit us, what they would have to say would definitely at least be worth listening too.
The Creator(s) manifesting right in front of you?
This would be perhaps the most convincing reason.
It would be hard to prove that any being is the creator, though. I mean, if this being could move mountains, vaporize oceans, destroy the moon and instantly re-create it, shut off our sun and start it again, read my mind, demonstrate perfect knowledge of humanity's history, while all the while coming up with reasonable answers to how he exists and how he created the universe and why, then it would be reasonable to believe it. It's possible that it could be an unimaginably advanced alien being (with, say, 2 billion years of additional technology under its belt), but if that's the case, this being is still absolutely worth listening to if it's so advanced and probably will understand the origins of the universe better than we would. And if it's lying, well, what reason would it have to lie to us when we'd be less significant than ants to it? So I'd say this would be the number one reason. I'd want more than just myself to see this being, though, or else there would be a high chance that I'm just loosing it. Schizophrenia isn't all that uncommon.
Other options could include finding a highly detailed message written in many earth languages within our DNA or something neat like that. None of them would be as convincing as an absurdly powerful self-proclaimed creator to me, though.
I would have a million and one questions for a supposed creator, though. If the creation represents the creator, I don't think I'd be all that fond of or impressed by the creator. I mean, did it intend to set up a system where life has to hunt, kill, and consume other life to survive? That's sadistic. Nature is cruel, with the weak dying and the strong surviving. Three year olds randomly get cancer and die. If the purpose of the universe is to support life, then this universe seems fairly bad at it with apparently all the other planets in our system without life (or at least non-microbial life), so much cold, empty space between everything, mass extinctions occurring so often, the existence of viruses, and so forth. If the purpose of the universe has nothing to do with life, then what's it for? To look pretty? Overall I think my assessment of the universe is that if it has a creator, it's probably more of a kid with an ant farm kind of creator.
Are Creationists right in that we Godless atheists can rationalise away anything they present to us?
I don't think so. In fact for years I tried to rationalize away the opposite point of view and failed at it.
-Lyn