One of the more difficult arguments to argue against in the theist arsenal is the "fine tuning" argument, i.e. there are 20+ physical constants in the universe, and if any of them were off by even a small amount, then "life wouldn't be possible". I've most recently read this argument in a book called "God: The Evidence: The Reconciliation of Faith and Reason in a Postsecular World" by Patrick Glynn, which I saw recommended somewhere and decided to buy.
It's a difficult read, since Glynn makes incorrect and blanket statements throughout when presenting the fine tuning argument, or, as he calls it, the Anthropic principle (which it actually isn't). The Anthropic Principle basically says that "of course we find ourselves in a universe that is compatible with human life, otherwise, how could we be observing it?", which is true.
Say we have a dartboard laying in the desert that is 1km by 1km square and a dart is dropped from 100km up. The dart's final resting point (assume it hits the dartboard) will be, essentially random. We can we say about where it landed? If you're a theist, you say "what are the chances that the dart ended up in this EXACT location? The changes much be billions, trillions to one!", which is true. However, you could say the same thing no matter where the dart hit. Without another dart, you can't say if someone was aiming or not. We just don't know. The same is true of the fine-tuned universe.
Glynn's presentation of the fine-tuning argument is awful. He makes statements like "the death of materialism", that one of the constants being different by even a small amount (strong force, weak force, gravity, or the relations between them etc...), then "life wouldn't be possible". He also generalizes this to say that the universe was created specifically to produce "human life", which is pretty silly. The vast majority of life is not human at all, so it's a weird statement to make. Clearly, it's not specific to humans at all, but humans can survive.
Also, I think he'd be much more honest to say that if these constants were different, then life as we know it wouldn't be possible. I can agree with that, but life wouldn't be possible? I am hardly an expert on life, but one thing that seems clear is that life is very adaptable and I couldn't count it out if gravity were slightly different. Maybe stars would have shorter or longer life-spans, or not form at all, but what does that say about what other types of life might be possible? Since we don't have any data points except our universe, and, in fact, our own planet, we have very little to say about this. We just don't know what type of life would be possible (if any is) if the 20+ constants were to be different. Maybe there is a large subset of the tunings that could produce some kind of life - we just don't know. To say otherwise, is at worst, a lie, at best, incompetent.
Not only that, but since we can't look at another universe, we have no idea if the constants can even BE different from what they are. Have we looked at millions of universes and seen that the constants vary in each and every one? No - we haven't. We don't know. It's an unknown that the fine tuning argument fails to address. We are, essentially, looking at one data point and fitting it to a curve. Something I can say to both sides of the argument, if you're making conclusions, you're probably wrong.
Now, let me put that argument aside, give ground and say that the fine-tuning argument is 100% correct. There WAS an intelligent force that shaped the constants and made them what they are. How do we conclude that the universe was made just for us, that human life is special and the tuning is what it is because God wanted humans to exist? I find this a very strange thing to say, considering that the universe is, essentially 100% lethal to humans (rounded to the nearest billion decimal places say). Only a very small portion of, one, tiny planet that we know of will support our type of life. A typical location in the universe is a hard vacuum that will kill humans instantly. It's not a friendly place for humans at all. Surely, this wasn't made for us. If we were to imagine a species that it WAS made for, I think something that could at least survive in a hard vacuum would be more likely. A space creature of some kind. If they said that their God made the universe just for them, we would have a hard time arguing that one.
So, a theist will argue their one data point, fitting the curve that they want to fit it to (NOTE: any curve fits a one-point data set), AND, the worst thing is that atheists often take this seriously. I've seen Daniel Dennett "call it a draw" in a debate against Dinesh D'Souza over the argument, and pysicist Lee Smolin suggest that universes evolve like any other life form and there may be an infinite number of them, and we just happen to find ourselves in that supports our type of life. Really? These concessions sounds like a couple of things: 1) like you are making shit up just because you are loyal to your atheistic position, think the argument has weight and are creating something equally ridiculous to counter it and 2) intellectually lazy and incompetent. The thing is, they are as scientific as science-fiction.
Where is the problem with saying "I don't know" or, better yet, "We don't know"? After all, it's the honest answer. We don't know. At best, we can show why the theistic argument is silly. BUT, this doesn't give us licence to make up stupid theories that sound scientific but are as pseudo-scientific as Intelligent Design. We can muse and even present these things in pop-science books, but to present them as scientific theories is dishonest. You need evidence for real, scientific theories and until we can see multiple universes, I think that's in short supply. Stop it. Stop it now!