Data is from here (I think): https://www12.statcan.gc.ca/census-recensement/2021/dp-pd/prof/details/page.cfm?LANG=E&GENDERlist=1&STATISTIClist=4&DGUIDlist=2021A000011124&HEADERlist=32&SearchText=Canada
I could not find who made the map.
That legend and colouring does not make sense to me 70+% catholic red, 20% protestant blue so, couldn't the shade area be done in either colour, and then you have the no religion one with similar opposite range.
I realize it says by largest religion but seems odd in some areas to have 20% as largest religion or non religion and the typical others altogether not making up enough to be 21%.