The following is a simple three step guide for constructive engagement with any proselytizing theist. It is, in effect, synopsis and clarification of an 'app' from this blogsite's main essay 'Truth?'. My hope is that by directing interested parties first to this shorter and more explicitly practical essay some may then become sufficiently intrigued to try the longer and harder one.
1. Inform the theist that he is offering proposals that you do not believe him to be able to qualify, through any coherent procedure for knowledge selection, as knowledge. Offer to open the debate with him at this level. Specifically, to work with him to arrive first at a procedure that you will both be able to understand to be capable of selecting proposals. Offer further that if this step can be completed then you will invite him to clearly state his theistic proposals for your mutual application of the procedure to them, and that if they then can be seen to qualify through it you will publically embrace them, on the spot. Explain politely that you do not wish to waste his time or yours on a further replay of the sterile debate that has been going on between our two sides for thousands of years. Simply, that if he is willing to match your level of commitment – in engaging within a format that you can both understand to be capable of clear and final settlement of our argument – then you are keen to talk to him. But that if he will only engage in the absence of any such format - to hold open his option for declaring a draw, through play of the 'faith' card or some related piece of hokum like 'warrant' or 'non-doxastic justification' as soon as he can see his position becoming rationally untenable - then you have other and more productive uses for your time. He can accept, or he can decline. If he chooses the latter, then it will be with pretty clear implications for both himself and those to whom he had been trying to propagate his theism. He will be openly admitting that he is only willing to play if he can't loose.
2. [He has accepted, and you’ve mutually agreed on a functional procedure]. Invite him to state his theism's defining proposals. Specifically, those which distinguish it from the all of the others, and from science (unless, of course, he would just as soon have you embrace any of those). Write his proposals down. Get him to sign the sheet. Because once you start applying a functional knowledge selection procedure to them – and they start to melt like sandcastles in the rising tide – he will expect to be able use the standard theist's dodge of linguistically obfuscating and morphing them to avoid your selection procedure’s hurdles. As in: “Well, I didn’t literally mean……..”, and “Of course '……..' should be understood only in a metaphorical sense”, and "How can we, as mere humans, hope to understand God's ways and motivations?", and all the rest of that ancient bag of tricks.
3. [You now have the functional procedure and the signed list]. Patiently and systematically apply the procedure to each one of his proposals. Show him how it clearly fails to qualify. Show him that any relaxation of the procedure’s tests that is sufficient to allow his proposals to qualify will also and simultaneously permit qualification of an effectively infinite number of other and logically exclusive proposals (including, and most naturally, those of science). So, and from which, he can see that his proposals simply cannot be coherently selected. That he can have them, but ultimately from precisely the same wishful-thought basis that a boy on a rooftop can have knowledge of his ability to fly like Superman after watching a Superman movie. Go ahead and put him explicitly in the position of having to publically renege on your agreement, or else renounce his theism. [Note: Our side has been in the position to do this for at least the past seventy years; but, in general, we have not. For some thoughts on how and why that has been, and whether our reticence has indeed been justifiable, please see another of the short essays at this blogsite: 'The Cuddly Kitten'.]
Primers for Step 3:
My main essay ‘Truth?’ (access via 'view my complete profile' at this blogsite).
William Clifford’s classic essay ‘The Ethics of Belief’.
George Smith’s ‘The Case Against God’.
Michael and Monier’s essay collection ‘The Impossibility of God’.
General recent writings of the ‘new atheists’ (Richard Dawkins, Sam Harris, Victor Stenger, Daniel Dennett, Susan Jacoby, Anthony Grayling, et al).