If I understand what you say about Bahnsen’s work, David, there would appear to be no possibility of a discussion between a believer and an unbeliever. Of course, a person may accept another’s presuppositions ‘for the sake of the argument’ with a view to examining their consistency or the validity of deductions from them without really accepting them. This is common practice. But to require another to commit to your presuppositions which are the very point at issue comes close to begging the question.
One might wonder if, in any rational conversation, there are always some presuppositions or other that are necessarily accepted by both interlocutors (a la Habermas) and if so, whether such commonality is enough to get a genuine conversation up and running.
That being said, exchanges between believers and unbelievers are often a conversation of the deaf (and not only in religious matters but in matters of economics, history, philosophy, climate change, and so on.)