Churches Stay, Believers Move: Four Censuses and OpenStreetMap History

Australia has thousands of churches. Some stand where their believers are leaving; others are missing where believers are arriving. We follow seven denominations — Anglican, Catholic, Uniting, Baptist, Presbyterian, Lutheran and Orthodox — through the four latest Censuses (2006, 2011, 2016, 2021), and compare them with the churches that volunteers had mapped on OpenStreetMap on each Census night. The first two and three-quarter minutes need no code. Then we open Python and R: five open downloads; OpenStreetMap history through the ohsome API; four Censuses on the same 2021 areas; a straight line per denomination; each region's share of churches against its share of believers, with a Poisson 95% range; and accurate maps with every church and the landmark churches. What the data say: • Uniting (−41%), Lutheran (−42%), Anglican (−33%) and Presbyterian (−30%) believers fell almost in a straight line (R² 0.85–0.96). Catholic (−1%), Orthodox (+3%) and Baptist (+10%) did not follow a line: they rose, then fell or levelled off. • The church map was filling in: 2,007 Christian churches were mapped on Census night 2011 and 5,320 in 2021, while NCLS estimated about 11,400 local churches in 2016. So we compare shares, not raw counts. • Fewer believers, many churches: mostly outside the capitals. In the rest of New South Wales six denominations have more churches than their share, while Uniting believers there fell by about 40%. Uniting in Greater Sydney: 90 churches where about 54 would be its share. • More believers, few churches: Catholics in Greater Perth — up about 10%, mostly in new outer suburbs, with 42 mapped churches where about 76 would be their share. • Steady believers, few churches: Catholics in Greater Sydney and Greater Melbourne have about 0.7 of their share of churches. • Checks: one request and seven separate requests give exactly the same counts; with the fuller 2026 map, or counting untagged churches, 24 of 27 results stay the same. Scope: OpenStreetMap had only about a quarter to a third of these churches tagged even in 2021 (Anglican 30%, Uniting 23%, Baptist 29% of the NCLS estimates for 2016), so raw counts mostly show volunteers mapping, and the share comparison assumes regions were mapped at a similar pace. Denominations are read from OpenStreetMap tags only. In the country people live far apart, so more small churches per believer is partly normal. A church on the map is not a congregation, and a region is not a person. Educational example of spatial analysis.