That's a tough one, I know what I might do/say, but also, for me that would greatly vary: depending on the tact being employed against me. Your particular views on religion, are in essence, very relevant, if the people engaging you are interested in a real and genuine exchange... although usually these sorts of so-called Christians, don't care about being convincing on an intellectual level, so much as they are just desperate to prove the efficacy of their gang-mentality, and the power of their brands' advertising campaign...
I don't know what your personal views are, but I would suggest that you communicate something along the lines of your reluctance to have conversations involving your important personal beliefs, at your place of work: or something along the lines of delineating a public/private life divide, analogous to the church/state divide.
Otherwise if your not afraid of using some of your privately held views to shield yourself from further, unwanted advances, you could suggest that your religious views are already settled, and offer some account of them, and perhaps offer up some sort of rhetorical question that makes their style of "faith" unpalatable to you, perhaps focusing on their feeling-based force of reasoning for their cause (an agree to disagree sort of tactic, because their beliefs are shrouded in unconvincing sentiments, that your just unable to swallow).
There are more variations of this sort of mixture, depending on how much more of your personal views you might want to publicly air in the context of ending these lines of conversation.
You could even profess agnosticism, and cite their kind of "faith" to be undeserving for some stated reason, that they cannot easily overcome— at least, without them having to sound like someone you might want to pursue in a deeper exchange (because they possess some particular knowledge your interested in, functioning as the cost of your invitation for such discussions).
In my experience, real openness, with clearly defined obstacles, is the better deterrent, that feels easier to maintain on the level of personal integrity.
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this is totally off topic, but I've recently been thinking on how to describe a good way to have an intellectual exchange..
instead of both participants trying to chair a discussion jointly, or wrestling for that tone of narration between the two of them: the best style of discussion: I would describe as taking turns to chair the discussion. So then each person gets a turn to chair the discussion, and draw the other through some sort of descriptive or illustrative gambit, for setting up their substantiating claims, or showcasing an issue of contention... the other party would make themselves subject to a level of participation, forcing themselves to answer the questions posed to them (if needed), or having some level of conceptual assent, in furtherance of the chaired discussion being checked up on during the exchange etc.
Of course, to save time, they could premise their participatory responses, with the respondent 'excepting' to the question's form or basis (i.e. discrediting or refuting the kind of substance being elicited by the prescribed participation)... Excepting can therefore be helpful to both parties causes, as it can be helpful for illustrating points which are to be made later on, perhaps even when the other side is given a turn to chair the discussion, but 'excepting' can even be helpful to the presiding chair, affording them the opportunity to clear up any potential, or even minor conceptual refutation, that might otherwise hinder the reception of the presentation that's being chaired. (So then both parties might even engage in a more equal, meta-exchange, even while only one of them is chairing the discussion— of course, the meta-exchange aspect, may be abandoned by one or both parties, each of their own accord, since exceptions may honorably take the form of broad sweeping negations based on divergent principles not yet in full view of the discussion, although without naming or at least alluding to the content of an 'exception', (or illustrating the insinuated kind of efficacy required to maintain such a reservation),— being followed up on, or interrogated about— would seem crassly intellectually-dishonest.
The above framework of discussion, does not immunize the possibility of having a disingenuous interlocutor, but this setup should work well enough to show an audience which side has more thoroughly workable rigor to his position or contention, at least if the one of them is capable of using the meta-exchange to complain well enough of sophistry (if one of the speakers is resorting to such low, an art).