Well, unlike the US, England (and the UK as I understand it) still has an established church, as do many other European countries, which may be part of the problem. In the US at least we can be more explicit about secularism, even though there has been a LOT of pushback.
The thing is, I’ve noticed that many westerners have no problem with secularism — until they run into religions and people who are “other.” Then, all of a sudden, it’s a bad idea. The connection between what looks like secularism and entrenched privilege for largely western cultural norms (which can be just as negative and arbitrary as anyone else’s) seems lost on such people.
One of the flip sides of secularism — at last coming at it from an American point of view — is that it means secularism for everyone. That means that if we are going to do work-arounds to accommodate people’s freedom of conscience, then that applies to everyone, whether you are Christian, Muslim, Jewish or Zoroastrian. The school cannot operate in a way that privileges one group over the other. It’s really that simple.
You can’t stop people from praying in school, but you can’t lead one (as a person in authority). You can offer space to religious clubs or groups but you can’t tell the Satanists or atheists they can’t meet in your gym while Campus for Christ can. You can’t tell students not to wear yarmulkes or headscarves or turbans, and you can’t have the 10 Commandments posted in the classrooms. People can do whatever they like within their religious community (within reason) but you can’t give it state sanction.
I don’t know why this is such a hard concept for people.
At least in the US, one of the reasons for an explicitly secular constitution is that there was a large variety of Christian sects in the US even then. Secularism was meant in part to leave room for that, so you wouldn’t leave anyone feeling alienated.
Let’s not forget that in 1780 the religious wars in England and Oliver Cromwell were not so far in the past — a century or so — and in fact much of the violence that we associate with Cromwell’s rule in Ireland spilled over into the Americas as well, though the targets weren’t Catholics. (In that case it was the Puritans, who supported Cromwell, engaging in battles with the people in places like Maryland, who supported the monarchy and its restoration). To say nothing of the then-current religious violence in Ireland. The founders of the US were not eager to repeat those experiences, nor create any more divisions between the colonies than already existed. Secularism was a principled position, but there was a very concrete political reason for it also.
When people feel alienated from the political process or the society in which they live, that’s when you get political violence (and religious violence). It is no accident that the countries that have a lot of terrorists and insurgencies tend to be ones that have managed to convince large sectors of the population that the ballot box and politics are ineffective. And that’s why secularism is so important to a modern — and yes, multicultural — society. You need to have a situation in which everyone feels like they are getting heard, where everyone has a stake in continuing some established order and method of resolving disputes. Otherwise yes, you do have whole classes of people who feel like they have no stake in the society at large, and that’s not good.