You presented your "spiritual vacuum" argument in an earlier thread, Khandro. It was the one where you mentioned Kilpatrick's "Christianity, Islam and Atheism: The Struggle for the Soul of the West". This book explains very clearly why Islamic ideology is incompatible with Western democracy. It is spot on, too, when it says that multiculturalism and cultural relativism have assisted the spread of Islam and blinded the West to the threat that that expansion poses. But I disagree with Kilpatrick's and your attributing the West's apparent commitment to cultural suicide to the rise of "secular atheism". The rise of Islam has happened not because the West has lost its faith, but because it has lost its wits.
Why and how the the elites of the West have come to embrace the self-contradictions of multiculturalism I don't know, but multiculturalism isn't something you catch when you become an atheist, although it's possible, I suppose, to catch atheism when you become a multiculturalist. Once multiculturalism is embraced an important critical faculty is lost - the ability to conceive that some cultures may be not only different from, but objectively superior to others, and that various cultures may be to some degree incompatible with each other. This in my opinion is what has caused the spiritual vacuum you talk about: "we" no longer have pride in our cultural inheritance, and having no pride in it "we" have nothing to defend, nothing to invite others to share. You call this apostasy; I call it la trahison des clercs. I am happy, however, to borrow the religious metaphor from Kilpatrick's title: the West has lost its soul.
I'm an atheist who is not part of the "we". Two other such are my favourite Spectator columnists Rod Liddle and Douglas Murray. But I think you know that.
What Khan's London will look like in four years time we'll find out in due course.