Just to help clarify; we are talking about a Gnostic (who view that matter is evil and the spirit is good, and it doesn’t matter what you do in your body as it doesn’t effect your spirit/soul?), not an Agnostic (one who is not sure that God exists)?
As in, the Gnostic gospels/writings the early church fathers rejected?
Would he describe himself as a so called 'Christian Gnostic?"
Gnosticism was perhaps the most dangerous heresy that threatened the early church during the first three centuries. Influenced by such philosophers as Plato, Gnosticism is based on two false premises. First, it espouses a dualism regarding spirit and matter. Gnostics assert that matter is inherently evil and spirit is good. As a result of this presupposition, Gnostics believe anything done in the body, even the grossest sin, has no meaning because real life exists in the spirit realm only.
William Lane Craig writes some fairly in depth thoughts about Gnosticism here:
So the atheist was trying to cause division between you and the other gentleman? No big surprise I suppose. Ask the atheist if they believe in a self-creating universe and perhaps share something like Lennox’s refutation of Stephen Hawkin from here: https://www.rzim.org/read/just-thinking-magazine/stephen-hawking-and-god such as this little snippet:
Take, for instance, Hawking’s statement quoted above: “Because there is a law of gravity the universe can and will create itself from nothing.” Clearly, he assumes that gravity (or perhaps only the law of gravity?) exists. That is not nothing. So the universe is not created from nothing.
Worse still, the statement “the universe can and will create itself from nothing” is self-contradictory. If I say, “X creates Y,” this presupposes the existence of X in the first place in order to bring Y into existence. If I say “X creates X,” I presuppose the existence of X in order to account for the existence of X. To presuppose the existence of the universe to account for its existence is logically incoherent.
Perhaps you could ask him what he thinks of the Cosmological Kalam arugment (https://www.youtube.com/watch?v=6CulBuMCLg0) or the Moral Law Giver (https://www.youtube.com/watch?v=OxiAikEk2vU)
… so back to the Gnostic. maybe ask him questions that lean towards the Moral category (How does he tell right from wrong?).
Maybe ask him “if Gnosticism is true, and it doesn’t matter what we do in our physical bodies, why is this not carried to our law courts as a legal defense?”. Ask about a hypothetical, but quite personal serious fictitious crime committed against someone he loves such as a close family member. Ask him, if the person who committed this crime (in the physical world) just brushes you off and says ‘oh I’m a Gnostic so it doesn’t matter what I do in the physical world’; does that sound like justice? does that hold water in our legal system? Ask him, if Hitler was a Gnostic, would that make him innocent?
just a few thoughts, hopefully others share something better…