Every dot is 1,000 people, the same number for every group, so two colours can be compared by counting. Fractional dots carry over to the next nearby area until there are enough people to draw one. Rings mark a body that is present but too small to reach one dot, or that reported congregations without a membership count. A ring says only "here", never how many.
Dots are placed at random within small census units, not at anyone's address. Where those units are built to a few thousand people each, as they are in the United States and Australia, that is already enough to put people about where they live. Where they are not, a population grid decides where inside the unit the dots fall, so that a large and mostly empty district does not wash evenly in colour. Either way it says nothing about where inside a unit a group lives.
Grey hatching marks places where people live and no source on this map counted them: part of a country its census or survey did not reach, such as Abkhazia, South Ossetia or Karabakh, or a country this map has no figures for. Hover over one, or tap it, for the reason.
Administrative boundaries come from the OCHA Common Operational Datasets on the Humanitarian Data Exchange and from geoBoundaries, both under CC BY 4.0, and coastlines from Natural Earth. Where a country's own agency publishes the units its census was counted on, those are used instead and named with the country. The population grid is Kontur Population under CC BY 4.0, and WorldPop under CC BY 4.0 for Eswatini. The basemap is OpenFreeMap, rendered from OpenStreetMap data. Each country's religion figures are cited under the title when that country is selected.