I'm trying to work on speaking better with laypeople about God and atheism.

 This is my attempt to convince a layperson that we can have ethics without God.  It is in conversation format:

I will set the fictional conversation between me and Jim, who is a minister that I debate from time to time on a different forum.

Jim:  If there is no God then morality is relative, conditioned by society. But if morals are relative and conditioned by society then murder is not absolutely immoral (even if it is illegal).  Therefore, either you must posit an absolute moral law giver, or you must concede that murder is not absolutely immoral.

DoctorO:  Jim, can I ask you a few questions?

Jim:  Sure, Doc.

DoctorO:   Firstly, let me ask you this.  If I said that Santa Claus is a man, would that be a true statement?

Jim:  Yes, Santa definitely is a man, and he wears a red suit.

DoctorO:  Okay, is Santa Claus a real person?

Jim:  Of course not.

DoctorO:  But you said that it is true that Santa Claus is a man and has a red suit.  How could he not be real?

Jim:  Well, Santa is imaginary.  We just assume that he is a man and wears a red suit.

DoctorO:  So, you're saying, that by definition, Santa Claus is a man who wears a red suit, but that doesn't mean he's real.  He's just imaginary.

Jim:  I suppose so.

DoctorO:  Okay, lets talk about cholesterol and heart attacks.

Jim:  How is that relevant?

DoctorO:  Hear me out.

Jim:  Alright.

DoctorO:  Would you agree that cholesterol in your arteries can cause you to have a heart attack?

Jim:  Yes.

DoctorO:  Why?

Jim:  Well, we have proof for it.  We know by science and medical studies that atherosclerosis can cause your arteries to be clogged, and they can stop your heart from working.

DoctorO:  So you need evidence that cholesterol causes heart attacks?

Jim:  Yes.

DoctorO:  What if I told you that since atherosclerosis in my arteries was a bad thing, so I'm not going to believe that I have atherosclerosis?  What if I told you that I'm going to eat Burger King or McDonald's every day because I can't get atherosclerosis?

Jim:  I'd tell you that you're stupid.  Whether or not you believe atherosclerosis will give you a heart attack is irrelevant. Regardless of what you believe, if you eat McDonald's every day, you'll probably have atherosclerosis and have a heart attack.

DoctorO:  So if the consequence of some event is bad, if I choose not to believe it will happen, it doesn't matter.  It will happen anyway, right?

Jim:  Yes.

DoctorO:  So lets talk about what you said about God and ethics.

Jim:  Sure.

DoctorO:  You said it would be bad if we didn't have God to give us ethics, right?

Jim:  I suppose.  If we didn't have an absolute law giver, people would be free to do whatever they want.

DoctorO:  So you just said before that simply because something is bad, my belief on whether or not it is real makes no difference.  It will be real just the same.

Jim:  Yes.

DoctorO:  Then your belief that ethics without God will cause bad consequences has no bearing on whether or not ethics come from God.  You want to believe that ethics come from God, because if they didn't, bad things would happen.  But we can't know whether or not God makes the laws, we can only say that if he doesn't, something bad will happen.

Jim:  Argh.  Well, God makes our ethics just the same.  He's our absolute law giver.

DoctorO:  Now you've told me that something bad will happen if we don't have absolute morals.  Why do you think that?  We said before that we must have evidence that clogged arteries cause heart attacks.  What evidence do you have that bad things will happen if we don't have absolute morals?

(This is the subject for another post that would be to long to discuss here.)

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DoctorO:  Now, Jim, you've said before that God is defined as the absolute law giver.

Jim:  Yeah, that's right.

DoctorO:  You told me a little bit ago that simply because Santa Clause is a man and wears a red suit, it doesn't mean he's a real person. 

Jim:  Yes. 

DoctorO:  So we can have a concept of an absolute law giving God in our heads, but what convinces you that he's any more real than Santa Clause, who is a man and wears a red suit?

Jim:  Well, we have the Bible and historical evidence.  Besides, I feel it in my heart of hearts that he exists.  I've been saved.  You can't tell me that wasn't real.

DoctorO:   So you agree then, that needing an absolute law giver doesn't mean God exists, you need other evidence to show he exists?

Jim:  Well yes.

DoctorO:  But, then Jim, you can't derive proof for God on the basis that we need him to give us ethics.

Let's sum it up, Jim.  First, we've seen how the first thing you have to do is prove that we can't have any ethics without God.  You haven't shown that.  You've said that we can't have "absolute" ethics without God, and I'm inclined to agree.  That doesn't mean we HAVE absolute ethics or that we can't have some other ethical concept.  Secondly, we've shown that disbelieving or believing in something because it has a bad consequence has nothing to do with whether or not its true.  I can disbelieve that my arteries are clogged, but that doesn't mean they aren't.  You can believe that ethics are absolute, because that would cause bad consequences, but that doesn't mean that ethics ARE absolute.  Thirdly, and finally, we can define the properties of some imaginary being like God or Santa Clause, but that doesn't mean that either exist.  We might have a concept of God being a divine law giver, but that belief is no more true in the real world as believing that Santa is a real person who is a man wearing a red suit.