Of course, it's all a matter of definition - and relativities. But because people with no religion are located mainly in countries and regions with declining populations then a certain Darwinian inevitability follows. The future belongs to those who show up and - other things equal - religious people have more babies than non-religious people. Even Darwin himself foresaw such a scenario (though he was a little less circumspect about the implications):
Thus the reckless, degraded, and often vicious members of society, tend to increase at a quicker rate than the provident and generally virtuous members. Or as Mr. Greg puts the case: "The careless, squalid, unaspiring Irishman multiplies like rabbits: the frugal, foreseeing, self-respecting, ambitious Scot, stern in his morality, spiritual in his faith, sagacious and disciplined in his intelligence, passes his best years in struggle and in celibacy, marries late, and leaves few behind him. Given a land originally peopled by a thousand Saxons and a thousand Celts—and in a dozen generations five-sixths of the population would be Celts, but five-sixths of the property, of the power, of the intellect, would belong to the one-sixth of Saxons that remained. In the eternal 'struggle for existence,' it would be the inferior and less favoured race that had prevailed—and prevailed by virtue not of its good qualities but of its faults." Charles Darwin, The Descent of ManThere is a fascinating interview with Tim Shah over at RN who criticises the whole 'God Is Dead' narrative of the New Atheists (and some rather old, rather dead ones as well for that matter). The rise of ISIS is a brutal reminder that religious belief (however distorted) still shapes world events for good or ill. An influence that looks set to increase rather than decrease in the decades ahead.
Demography may be destiny but it is not inevitability (even Darwin saw some inevitable checks on rabbit-multiplying Irishmen: like scarce resources). Nor is religious identity necessarily an inevitable influence on belief let alone behaviour. One of the best reports on the state of religion in the world today is the Religion Monitor by the Bertelsmann Foundation. It drills into the 'spiritual but not religious' meme, as well as the influence of religion on people's daily lives, families, politics and much more besides. There are as many if not more 'cultural religious' than 'believing/practicing religious' in numerous parts of the world. While more people in the future may self-identify as belonging to a particular religious group, their behaviours may be indistinguishable from the diminishing band of non-religious in their midst.
I think this is especially relevant to Ireland as we move rapidly towards the orthodox, Western model of secular progressivism. Sure, we still like our Communions and Confirmations, but I would hazard a guess that we will follow the UK path in relation to religious belief, absent any major cultural differences (long story short: there aren't any anymore).
Crucially, the Pew projections indicate that Islam will be the fastest growing religion in the foreseeable future - which suggests that religion will be more important, not less important to the course of the future. Already, large minorities of people in non-Islamic countries view Islam as incompatible with the Western World:
How such perceptions will change - or harden - in the decades ahead will make sure that religion will remain central to the course of history and of the future.