I think I understand your strength of feeling, but many British people will have read the argument of the popular historian
@holland_tom that secularism, as a separation of religion and politics, the sacred and the profane, the limit of human memory and the eternal, is largely a product of Christian civilisation.
The argument seems to be that ideas like "give unto Caesar what is Caesar's and unto God what is God's" and "the Kingdom of God is not of this Earth" were 'acorns' that grew into this separation.
My understanding is that he argues that even belonging to a "religion" is a product of Christian civilisation, where previously most people would have more simply thought of themselves in terms of their lineage and would have made due reverence to a God or gods was in which doubt played little part.
If there is anything to this perspective then perhaps there is not so much a collapse of the sacred into the secular for the first time, unlike any other religion, but instead a complete abandment of the Christian sacred and a move to the worship of something else.
However, I suspect that Tom Holland would argue that we are going through another reformation of Christianity with its fundamental assumptions unchanged.
Word has just come down that St. Paul’s Cathedral in London—the second most important church in the Anglican world—has imitated Canterbury Cathedral in permitting “raves,” nightclub-like entertainments, to take place in its confines.
The usual reasons for acquiescing to this have been trotted out: reaching out to the young, trying to get with the times, etc. But this development simply shows that the leadership of the Church of England has lost any sense of the crucial distinction between the sacred and the secular.
Across the centuries and across the cultures, our ancestors seem to have intuitively grasped the importance of this demarcation. Whereas the secular is the arena of ordinary experience and activity, the sacred is another realm, a different dimension.
Accordingly, in the properly sacred space, one is lifted out of the conventional and introduced into the numinous. This is why Christian cathedrals are filled with images of the saints and angels and why their interiors are meant first to disorient us and then to reorient us toward the divine.
It is also why, in the initiation rituals of primal peoples, a young person is purposely ripped out of the ordinary and introduced into the mythic and mystic dimension. And it is why, throughout the Bible, sacred spaces are carefully set apart from common spaces.
The contemporary West is, quite literally, the first culture in human history that is in danger of collapsing the sacred into the secular, or better, losing any sense of the difference between the two.
This is doing enormous damage, especially to young people, whose lives have been flattened into a preoccupation with money, worldly success, and material things.
The Church indeed should go out into the secular space to find those who have wandered away from God, but it should then lead those wanderers back to the holy.
Having “raves” in the nave of one of the most sacred spaces in the Christian world is, I’m sad to say, another sign of our pitiful surrender to ideological secularism.