I hope you'll forgive me if I eschew the customary Trinity Sunday joke about celestial mathematics. It's usually made in a rather apologetic fashion to give the impression that the preacher isn't too bothered about all this high-flown doctrine, and is much more interested in more practical things, like being nice to old ladies. Even some of my fellow ordinands dread the prospect of having to preach on Trinity Sunday. They see it as a dry, doctrinal affair compared with the preceding drama of Eastertide. And to some extent, they are right: this is the odd one out among the feasts of the Church, in that it is the only one which is based not on an event or a person, but on a doctrine. As such, it is condemned for one of the greatest sins of modernity: not being 'relevant.'