I would agree with Crane, it’s just a matter of having the right person get into the right position and it can all easily go sideways.
In Japan, for example, the population love peace, are glad that they have no appreciable military and that it’s not allowed to engage in battle, and would never allow the government to create nor purchase nuclear weapons.
The government, on the other hand, is formed almost completely of nationalists who wish they could form a strong military, build nukes, etc.
Turkey worships Kemal Atataturk, just as we do so for George Washington, and he was a devout secularist and championed it strongly as one of the founding principals of the nation. Yet, despite that, Erdogan has successfully used the Islamic voting block to power his way to absolute power, which has meant the ability to rewrite the makeup of the government, fill key government roles with loyalists, and adjust voting measures to ensure continued power.
While it may well be that religiosity is on the fall, if you get someone in power who can suppress speech effectively, change school doctrine to include religious materials, and rig elections, then they would be able to maintain power for a Christian party and see that it lasts long enough until the population was forced back into widespread belief.
A minority group can take control over a country and force the rest of the country to go their way, if they are dedicated enough.
I would also worry about the continued irreligiosity of our youth. The Internet allowed atheists and agnostics were a majority of the first netizens and they influenced a lot of the new users to the Internet. But now the Internet is starting to allow communities to go off and dive into their own navels with impunity. The Internet is may continue its plunge into empowering stupidity, craziness, and woo, rather than hindering it.
And also, people tend to go back to the way that they were raised once they settle down and get married. I’ve had occassion to witness and hear about many a relationship with a person from Asia and Latin America and where the man/woman will initially seem progressive and American, suddenly things start to get serious, they tie the knot, or whatever and suddenly everything from the home country comes back, deep religiosity, horribly conservative values, etc. So while it could be that all the 20-somethings are currently living it up and championing LGBT rights and so on, they could hit 30 and go back to being the Christian fundamentalists that they were raised as.