Two answers to my own question (sorry, it's an irritating rhetorical tick, I know, but somehow one has to get started...)
Firstly, I'm not a secular, or humanist, or anything else, celebrant. I'm a funeral celebrant. The whole belief/non-belief thing is, I feel, a huge red herring. (Red basking shark?) My beliefs are usually entirely irrelevant. I will do what a family wants. If they asked for a priestly function such as a formal blessing, which would make me feel inauthentic, I would ask them to call for a priest.
I'm not an ordained minister, I can't and shouldn't do "In the name of our Lord Jesus Christ.." etc. The issue hasn't arisen yet because I'm usually simply asked to organise a hymn or two, or lead The Lord's Prayer. People have the sense to ask for a priest if they want one, and someone like me if they don't. Last week an out-and-out atheist, next week a "possibilist," week after a non-Church-going sort of Christian, week after a mostly-ex Muslim who wants some religious music played. Fascinating work, no room for useless polarisations. What I believe in, as a celebrant, is good funerals.
Secondly, because this bloke:
has written a really useful book, which I also thought was a very good read.
He explores, in an entertainingly firm, conversational way, why he finds it makes emotional sense for him to be a Christian, and he comes at you with all guns blazing if you try to hide behind the usual liberal laziness. He's very cross about the way our increasingly secular society responds to Christian beliefs. He makes me feel that he is often right to be cross.
There's a bit where he is sitting in a church contemplating - well, infinity, I guess - looking for the presence of God - which is a terrific read, and relates, for me, to a mindful awareness of the universe in the present moment.
There's another bit where he re-tells the story of the woman taken in adultery - the one they were all about to kill by chucking stones at her - which I found intensely moving.
His is not the voice of the apologetic fence-sitting cleric, nor the wearying clamour of the evangelical bore. He just goes straight for it in a way that certainly made me, a non-Christian in most senses of the word, sit up and think.
One of the things that drove me away from Christianity when I was a lad was the teacher, allegedly a Christian, who looked for an easy rationalist excuse for miracles. Walking on water? Lots of sandbanks in the Sea of Galilee. Burning bush? Spontaneous combustion of subterranean methane. Oh, FFS, as we say these days, either they were miracles or they weren't. Francis Spufford is:
I salute him for it. Don't sneer at Christians unless you've read this book. (Well, don't sneer at them anyway, it's not nice - but you know what I mean!)