Religious life is about something real in human experience that is not constrained by what Wittgenstein called 'all that is the case'. In this sense Heidegger is not simply 'mistaken' - he just asks us, as philosophers mostly do, to think more carefully about what we're saying.

Barth's approach tears up any possibility of dialogue between faith and unfaith or between theology and other human sciences. Theology just says what it says on the basis of scripture, and that's that.

But, inevitably, as he [Kierkegaard] approaches what we might call his Christocentric climax many readers drop off. Many scholars just leave that part of his authorship alone.

Ethics arises in the recognition of our obligation to care for others as beings, like us, exposed to mortality - that is, beings who need our help. Buddhism, not wrongly, extends this to 'all sentient beings'.

But why should a religious person be interested in a work like Heidegger's that many regard as the epitome of nihilism? For a start, because Heidegger forces us in a way that few philosophers do to really think through the seriousness and all-encompassing nature of our mortality.

Perhaps unsurprisingly, there's a paradox here! Kierkegaard's own indirect communication proposes that we start with the experience of those who don't believe and meet them on their own ground. His success in doing this is evidenced by the fact that, at least for some periods of the 20th century, aspects of his work became a major focus for radical thinkers of various kinds, including the non-religious and, interestingly, a significant number of Jewish thinkers (Buber, Rosenzweig, Taubes, and others).

In a sense these are questions that most people ask themselves to some extent. They become philosophical when asked with a persistence and rigour that pushes past conventional or evasive answers. It's nothing to do with acquiring a technical facility in an academic discipline.