In theology, there are certain terms whose meanings are kind of taken for granted by the vast majority of people, without them ever being actually defined in the conversation. Yes, often even by skeptics/atheists.
It is, in fact, a good exercise to force the theist to define their terms before proceeding in the conversation. For example, whenever they use the term "soul" or "spiritual," try immediately stopping them and asking for a definition. "What do you mean by 'soul'?" "What do you mean by 'spiritual'?"
In surprisingly many cases they will actually be unable to come up with a concrete and coherent definition. In an online conversation I asked what they meant by "soul," and the only answer I got, even after asking several times, was "you know what I mean." No, I don't know what you mean, please explain it to me, I responded. I was not just playing dumb. I was honestly trying to get a coherent definition of "soul" so that we could proceed the conversation with a clear definition. The other person was unable to give any kind of definition.
Most theists take terms like "soul" for granted, as if it was completely clear and everyday knowledge what that term means, when in fact it's not clear at all, not even to them. "Spiritual" is another related term that's likewise taken for granted.
Of course even in the case where they can give some kind of coherent definition, you can then proceed to ask them how exactly do they know that such a thing even exists. (Although, according to my experience, from that point forward the conversation will typically start going in circles, jumping from one thing to the next, and coming back to the same thing again and again... This usually leads nowhere, and trying to break the circle can be really difficult. But such definitions can be a good starting point for an actual discussion nevertheless.)