I can see what you mean about not wishing to endorse the religion of the church, but if the concert is nothing to do with that religion, the church becomes just a building, doesn't it? And one with great accoustics, probably.

In England at least, churches are also a big part of our heritage. I go into our village church sometimes (not for worship) and as I stand there I know that this building, or parts of it, has been there for almost a thousand years. It will have seen various forms of Christianity practised there, pre-and post-Reformation, but it will also have been a place of comfort and sanctuary in times of war, famine, and plague. It will have seen celebrations of thousands of births, marriages, deaths, and all sorts of unofficial social rituals will have taken place there too.Its bells and organ will have been familiar sounds to generations of villagers.

I enjoy this feeling of history and tradition, and don't even mind contributing to the maintainance of the building, although I don't wish to support the work of the C of E.

And as a humanist and atheist, I like the idea of using the church building for a positive and worthwhile secular purpose such as a music concert. Even if some of the money goes to keeping up the building, is that a bad thing?