Many religions, especially those with Mosaic/Abrahamic connections, embrace the concept of ‘tithing’: giving one tenth of your income to the church. Originally, of course, this referred primarily to agricultural produce; in more recent years it tends to focus on Standing Orders and Gift Aid. Jesus himself apparently said little or nothing about it, which has allowed different groups of his followers to put differing degrees of emphasis, or compulsion, on on the concept since. But it is, of course, generally regarded by church leaders as desirable, and in my youth, sermons pointing out the biblical distinction between ‘tithes and offerings’ were not uncommon — the former being expected and, in effect, already belonging to God; the additional voluntary contributions constituting virtue (as long as both were done in secret).
Over coffee, after one of these sermons, I sympathised with the preacher. How obvious it was, I said, that very few people took these biblical directives to heart. He made some remark about how he was fortunate to have a generous congregation, and I fear I may have emitted something like a snort as I looked around at the gathered multitudes. I was very fond of them too, and many of them were very generous people, but I pointed out the mathematics…
In the Anglican church, for example, if people took tithing seriously, then every nine parishioners could support one vicar, who would then have the same average standard of living as his flock. (That got his attention!) You would need ten in your congregation if the vicar tithed too (which I guess would allow ten vicars to support one bishop, and so on).
OK, and you have to maintain the church buildings, do some good works, etc. So perhaps you need a congregation of about 15 people per priest for the church and clergy to lead a comfortable life. (That’s assuming your church doesn’t have any other income from land, bequests, indulgences or whatever…)
In the 35 or so years since that conversation, church attendance has declined so rapidly that many parishes will be doing this kind of maths for themselves. The average C of E congregation is something like 30-40 at present. But back then, though, I fear I may have left him contemplating his congregation of around 150 a bit less favourably than when he started his mug of coffee.