It's all too common for atheists to encounter theists who are more interested in sermonizing or lecturing than in conversation. There are ways to prevent this from happening before it starts. You can tongue-tie a theist in a way that will make discussion possible if that's what they want but prevent preaching if that's what they intend. Life is short — too short to waste time being preached at for no good reason. Here are some questions which may make it difficult for preaching and lecturing theists to continue.
Are You Here to Preach or to Talk?
There's nothing wrong with asking a person to state their intentions clearly and there's nothing wrong with wanting to limit contact with people who are only interested in preaching. The difference between conversation and preaching is crucial: a conversation or discussion is a dialog where everyone learns and everyone is willing to modify their views based on that new information. A sermon is where one person speaks without listening and with the intention of teaching rather than learning.
Someone who intends to preach will be caught between a desire to be honest and their concern that honesty will mean that you walk away. You should make it clear that such concern is justified and that you'll walk away from preaching. Force them to choose between preaching to empty air and real conversation.
Can You Define God?
You can't have a substantive discussion about "God" or the existence of "God" unless everyone is clear on what they mean by "God" and everyone uses "God" in the same way. But when was the last time you saw that happen? In my experience, Christians tend to object to being asked to define clearly and coherently what they mean by "God," preferring instead that atheists simply assume that we know what is meant.
Don't play that game; an unstated definition is too easy to modify every time an objection is raised. Insist that there is no point in continuing unless and until the believer makes clear what it is exactly they claim exists. If they won't invest effort into that, why should you invest effort into examining that claim?
If they can't or won't define what they mean by "God," there's no point in continuing — and if you insist upon it as a prerequisite, you won't have to waste any time because those who can't define "God" and can't explain what they are talking about will just sputter.
What Would Qualify as Disproof?
If a theist is genuinely interested in discussion, then they must be open to learning something new — and in that case, they must be open to modifying their current opinions in the light of new information. So it's reasonable to ask them to have some idea of what might qualify for them as disproof of their god... or at least a reason for them to reconsider their belief. They don't necessarily need something ultra-speciific in mind, just some conception of what would do it.
If in contrast they are convinced that nothing could ever get them to reconsider their theism, then they are saying that they are not open to change and not open to learning something new from you. That means they aren't interested in a real discussion; instead, their intention is to preach. In that case, tell them that you just aren't interested. If you insist on this as a prerequisite for discussion, you'll weed out the un-serious theists because those who can't imagine ever being proven wrong will have nothing to say.
If an Argument is Proven Wrong, Will You Reconsider?
It's reasonable for theists to ask you the same thing: what would convince you to change your mind or at least reconsider? You can't answer that immediately because it all depends on the exact nature of their claim, which means it depends on how they answer the question "how do you define 'God'?" Only when they can define what they are claiming is there any chance that you can offer ideas for what might change your mind.
In the meantime, you should insist that they only offer arguments that they themselves find compelling. This means that if the argument is proven wrong, they would have to reconsider their position. The alternative is that they don't really care about the validity or soundness of their arguments — and if they don't care, why should you?
Why waste time evaluating an argument if the theist wouldn't reconsider no matter how bad or wrong the argument turns out to be. If they have no such arguments, then their position isn't based on reason and evidence, in which case, you have no reason to take it seriously.
If an Argument is Proven Wrong, Will You Stop Using It?
If a real conversation depends on learning, then that means learning which arguments can be used and which should be dropped. A sincere and honest person will stop using arguments that have been proven wrong or fallacious; an insincere person who cares more about converting others than about truth will just keep using whatever arguments sound good to them, regardless of their soundness or validity.
You should thus ask a person in advance whether they are willing to drop invalid and unsound arguments entirely — not just in conversation with you, but from any future discussions as well. A person who can't promise that is a person who intends to just preach rather than have a substantive conversation. Insisting on this as a prerequisite means weeding out those who are beyond reasoned conversation.
Will You Conform to Principles of Logic & Reason?
There's no chance a serious conversation unless everyone is willing to accept independent standards of reason and logic. If one side eschews those standards, then no real conversation is possible. Indeed, anyone who rejects them might as well just be spouting random statements in a foreign language for all the value their voice will have. It's crucial, though, to get a person's agreement to them in advance because so few understand what they entail. The bare minimum a theist must be willing to accept if they are going to have a conversation instead of preach is:
- Whoever makes an empirical assertion has the initial, primary burden of proof or support
- Empirical assertions require empirical evidence as support
- A proposition with greater supporting evidence should be accepted as being more likely true
- One disagreement or argument should be resolved before moving on to another
- Errors or flaws in an argument should be admitted before moving on to another
A person who can't accept these minimal standards of reasoned discourse are incapable of reasoned discourse and is, at best, only going to preach. Demanding them as a prerequisite for any conversation will put a theist in a difficult situation if they have no desire for dialog and no ability to reason.