Secularisation is the key word, but, as Damian says, it requires 'unpacking.'
"The deadliest enemy of western Christianity is not Islam or atheism but the infinitely complex process of secularisation.
Or, to put it another way, choice. Long before digital technology, social mobility was undermining what the American scholar of religion Peter Berger calls ‘plausibility structures’ — the networks of people, traditionally your family, friends and neighbours, who believe the same thing as you do.
I’m not saying that my Catholic grandparents accepted the doctrine of transubstantiation only because the people closest to them shared that conviction: faith can’t be reduced to social processes. But supernatural belief is hard to sustain once plausibility structures collapse.
You go away to university and suddenly almost nobody believes what you do, or did. Your siblings move to different towns, so you won’t see them in church any more. Your laptop plugs you into any social network that takes your fancy. Even if you’re born again as an evangelical Christian, life pushes you from one congregation to another. Many Evangelicals get bored and turn into nones."
In other words, secularisation is the phenomenon when faith slips out of one's life because other beliefs - newly plausible explanations for the the point and purpose of living - creep in. Loss of family structure is arguably a critical element in widespread loss of Christian plausibility - a loss that may come through moving from one place to another, or through marriage breakdown.
Thompson also makes the point, vital for churches to understand, that secularisation does not only happen through forces external to the churches. It is creeping into the churches. (In the British context in which he writes, Thompson, a conservative Catholic, is fiercely critical of secularised Catholicism).
"It can’t be stressed too often that the secularisation that happens inside churches is as important as the sort that happens outside them."
Indeed, to go back to that other S word which troubles the churches of the West today, sex, many of our debates are precisely because of the infection of secularisation dis-easing the churches about beliefs which formerly were plausible to all and now only seem plausible to some.
Within the British scene, Thompson is pessimistic:
"James Davison Hunter, an orthodox Christian, believes he has found a way out of this maze: follow the instructions of Jesus and ‘faithful presence’ will change hearts, if not society. This seems to me to ignore the reality that religions invariably die, at least on a local level, when no one can be bothered to attend their services. As a Catholic, I believe that the gates of hell will not prevail against the church founded by Peter. There will always be someone to take the place of ‘the last Christian’. But not necessarily in Britain, where the death rattle has begun."
Now, let me conclude on a pessimistic note! If Western Christianity dies out, we Western Christians have the consoling thought that Christianity will yet live ... in the East. But how long will faith last there, between the squeeze of growing militancy among Muslims and the rise of Western style secular plausibility as materialism sweeps across the planet?
OK. Let's not conclude there. If there is a one word description of our greatest challenge, secularisation, there is a one word response in respect of Who holds the key to a new future: God.