The origin of species
This is a popular question. It comes about because the theist has been taught a theory that is nothing like evolution, and wants to explain that it's rubbish. The theory they want to discuss is indeed rubbish and is only vaguely on nodding terms with evolution. I shall call this theory "not-evolution".
Here's now not-evolution works. One day, a monkey gave birth to a human. Or possibly what happened is that suddenly, all monkeys were giving birth to humans
Yes, it's absurd. Or a squirrel gave birth to a dog. Or something. I have no idea where they learn the theory of not-evolution; perhaps in their religious school?
So now you have a problem. If you try to explain the actual theory of evolution, then you're trying to put a several-hours tutorial, into a short time, which is probably impossible, and even if you do, you may encounter the phenomenon of "Invincible Ignorance" (I'll explain that later). Your best bet is to recommend a website that explains it, and then to recommend a good book. Because trying to explain evolution to someone starting off with not-evolution, is like trying to explain Special Relativity to a goldfish.
And a classic symptom of not-evolution is "If people came from monkeys, why are there still monkeys." You can reply "If Mormon came from Christianity, why are there still Christians", but I doubt if that will do any good.
Another classic symptom is "They haven't found the missing link". Actually, they have.
Or "There's no proof" Actually, there is.
Or "It's just a theory". No, it's a fact.
This is actually a theological term. "Vincible ignorance" is ignorance that can be overcome with teaching. For example, if you didn't know that there are 5240 feet in a statute mile, then that is "Vincible ignorance" and easily dealt with.
"Invincible ignorance" is when, after you have been told that there are 5240 feet in a statute mile, and it has been demonstrated to you, then you continue to insist that it's only 5000. Yes, I know it sounds ridiculous, but invincible ignorance really is a thing. I've met it a few times.
I know of no cure for it.
The origin of life
Many theists accept evolution (including the Pope). But then comes the question of, before there was life, there wasn't life. How did life originate, or to give that its fancy word "Abiogenesis".
My first explanation is "I'm not a biochemist or biologist, and I don't know, but I do know that there are several possibilities, and research is still proceeding. Would you like me to list some of the possibilities?" If they do, the Wikipedia again. I give them the first one, then ask if they want some of the other alternative possibilities.
The origin of the universe
Sometimes this starts off as "How did the planet Earth form" which is an easy one, but after that is answered, you get asked the origin of the universe, so it's easiest to go straight to that. And it's a doozy!
The Big Bang is actually not really relevant, because that begs the question of where the matter in the Big Bang came from.
Again, I start off with I'm not a cosmologist, and I don't know, but I do know that there are several possibilities, and research is still proceeding. Would you like me to list some of the possibilities?"
And then I list the following.
1) It was always there.
2) It came from nothing.
3) It was created by Leprechauns, who have now departed and gone elsewhere
4) It was created by a unicorn, but the unicorn was used up in the process and no longer exists.
5) It was created by a Fairy, and the Fairy is still here, but doesn't do anything now
And then the big discussion starts. I try to get them to deal with one at a time. The usual response to 1) is incredulity, but incredulity butters no parsnips. Why can't it have always been there? And sometimes I might remind them that this is exactly what they believe about their god. So they might say "But everything has a beginning" and I say "Well, maybe except the universe".
The usual response to 2) is also incredulity. How can you get something from nothing? There's a couple of responses. The first is "How do you know? Have you ever seen 'nothing'? I haven't. "Perhaps one of the properties of nothing is that it always spontaneously turns into "something". The second is, actually, quantum mechanics says exactly that. If you have empty space, then it will immediately contain something. And the third is to recommend a book by Lawrence Krauss "A universe from nothing" which shows just how a universe could come from nothing.
I've never gotten as far as 3), 4) or 5).
This could be as simple as the "Goldilocks" ideal, or more universal. The Goldilocks idea is that planet Earth is in the ideal zone for life. How could that be just chance?
That's easy to deal with. Here we are. If Earth hadn't been suitable for life, we wouldn't be asking this question. Maybe there are zillions of other beings on their planets asking the same question. And clearly, on a planet that is completely inimical to life, there's no-one to ask that question. So we can deduce nothing from the fact that we live on a planet conducive to life.
More difficult is the same thing applied to the universe. Constants such as the ratio of the gravitational force to the electromagnetic force, and several dozen other such constants, are just right for life to happen. Surely the odds against that are astronomical?
There are two possible answers, but the main answer is "I'm not a cosmologist, I don't know." After that, there's the fact that we do not currently have the ultimate theory of physics (and maybe never will). It might seem strange that if you dissociate water into hydrogen and oxygen, there is *exactly* twice as many hydrogen as oxygen atoms. Not 2.00001 or 1.99999, it's exactly 2.00000. The odds against that are astronomic, surely? No, not if you go to the underlying theory; each water molecule is made of one atom and two hydrogen atoms. So the ratio of 2:1 *has* to be exact.
Likewise, perhaps there's an underlying theory that we haven't yet understood that forces the constants of the universe to be what they are. We don't know.
The other possible answer is the multiverse. This relates to the "something from nothing" idea. Perhaps universes are spawned all the time, in vast numbers, out of nothing. Some immediately collapse, some don't have constants that allow star formation. But some are such that it's possible for stars to form, and planets, and life, and on those, it's possible to have beings that ask questions like this.
The parable of the diamond
A common statement by theists is "My religion comforts me. What harm is there?"
First of all, if you keep it to yourself and don't try to run my life by your religion, there would be no harm. But people use their sincerely held religious convictions, to try to run other people's lives. Gay marriage is a good example of this. Theists like to forbid other people marrying who they choose, with consenting partners. My view is that if a theist doesn't want to marry someone of the same sex, that's fine by me, but just as I would not tell any theist who they may or may not marry, so I would not want any theist to tell me. The harm is when theists try to pass laws that make their religion or religious views compulsory. The harm is when they require people who don't subscribe to their religion, to support it financially.
And for comfort, here's the parable of the diamond. A man says that in my back garden, there is a diamond the size of a hen's egg. I haven't actually tried to dig for it, but my belief in its existence comforts me.
And if you want a painkiller, there's a good case to be made that a bottle of whiskey does less damage than religion. I would recommend "God is not great - how religion poisons everything " by Christopher Hitchens.
I've not seen this used, but the technique is to present "fact" after "fact" at a galloping speed. This is effective in a time-limited debate before an audience, because it isn't possible to answer every point. Or even remember them. The audience is left with an impression of, on one side a mountain of evidence, on the other side a few responses.
But in conversation, my response would be to wait until the gallop ends, then debate the first point, and not leave it until that point was finished (which it wouldn't be) then move on to the second, and so on.
This says that since hell is so awful, it makes sense to believe in order to avoid that fate. The first answer is actually I don't have conscious control over my beliefs. My beliefs are swayed by evidence. So, for example, it would be great if I could fly, so why don't I stand on a hair and jump? Because even standing on my chair, I still believe that gravity will make me fall, and I cannot force myself to believe otherwise. What I could do, is pretend to believe. But will that fool your god?
The second answer is, you think your god's hell is bad? There are 42,000 other gods, and some of them have an even worse hell. So are you suggesting that the wise course is to believe in the god that has the worst hell?
The Kalam cosmological argument
This is an egregiously specious argument, and to be fair I've never had it used on me. It goes like this.
1) Whatever begins to exist has a cause;
2) The universe began to exist;
3) The universe has a cause.
The problem is, you haven't demonstrated 1), you're just assuming it. And you're assuming it in the teeth of the evidence from quantum mechanics. It just isn't true.
And you haven't demonstrated 2). Maybe the universe was always there.
And even if you start off from 3 as your initial axiom (which you'd have to do because the Kalam doesn't prove it) then how do you know that cause is still in existence, that the cause is a god, and that the cause gives a monkey's about you and me?
Total non-starter. Maybe that's why I haven't heard it. Or maybe I haven't heard it because whenever anyone gets as far as 1) I dispute that assumption, so the argument can go no further.
The Ontological argument
This is another attempt to logic a god into existence. Shout -out to Anselm of Canterbury.
1) By definition, God is a being than which none greater can be imagined.
2) A being that necessarily exists in reality is greater than a being that does not necessarily exist.
3) Thus, by definition, if God exists as an idea in the mind but does not necessarily exist in reality, then we can imagine something that is greater than God.
4) But we cannot imagine something that is greater than God.
5) Thus, if God exists in the mind as an idea, then God necessarily exists in reality.
6) God exists in the mind as an idea.
7) Therefore, God necessarily exists in reality.
Well, I can't argue against 1), if it's a definition. But I disagree with 2). A being that doesn't exist is greater than one that does exist. Consider, for example, two beings. Sally, does exist and baked a cake. Penny doesn't exist, and nevertheless baked a cake. Since baking a cake when you don't exist is vastly more impressive than baking a cake when you do exist, Penny is greater than Sally.
And so 3 to 7 are floating on a proposition which isn't true. And finally, even if you assume that 7) is true, then how did you determine that it gives a monkey's about you and me?
The universe is God
I've heard this suggested. when you drill into it, it turns out that he's decided that the universe and god are synonyms. Fine; I also believe that the universe exists. Do you think that the universe can answer prayers, or cares about you?
There are similar notions; nature is God, the unified consciousness of humanity is God. I tend to smile, not and back quietly away.
What would make you believe?
I think this challenge is designed to short-cut the process of turning an atheist into a believer. My response is "Convincing evidence", which is a no-brainer. If it convinces me, then I'm convinced. Duh. But specifically, what? I don't know. But if your god is omniscient then he'll know what would convince me. And if he's omnipotent, he should be able to say "Hi" to me.
To believe, you must open your heart.
Here, let me summarise that for you. To believe, I must first of all, believe.
You can't prove God doesn't exist
There are a few good answers here.
1) You're the one making an extraordinary claim. You're not just claiming that there's a pencil on your desk, which I'd believe without any more than your say-so. The burden of proof is on the person claiming something extraordinary, which means you.
2) Yes I can, and there are umpteen proofs that various gods don't exist. My favourite is the Quantum proof (see below).
Look, if you genuinely believe that Matthew 21:22 (and several other places) that "If you believe, you will receive whatever you ask in prayer.", then how about you just pray me into becoming a Christian. Let me know when you're done, and I'll let you know if it worked.
Quantum Mechanics is a favourite of several woo-purveyors, such as Deepak Chopra, who seem to drag the word "quantum" into every third sentence. I think the idea is that QM is so difficult to understand, that it must be connected to other things that are difficult, like theology. Also, some people have gotten the idea that consciousness is one of the ingredients in QM (which it isn't).
So I was thinking about that one day, and I came up with a proof that no omniscient (or omnipotent, or omnipresent) god can exist, because of the double slit experiment. And I called it "Quantum Theology", but I'm sad to say that other people used that phrase before me.
Here's a good one. There were thousands of martyrs for Christianity. They wouldn't have let themselves be martyred unless they sincerely believed, would they?
Well, maybe they did sincerely believe, but that doesn't make it true. And what did the 9/11 suiciders believe in? It certainly wasn't Christianity.
Two billion people believe in Jesus. That makes it true.
No it doesn't. five billion don't believe in him, so if you want to go by voting, Christianity isn't true (and the same argument works for other religions).