Population doesn't spread religion faster, though it should.
The best way to spread a powerful religion without too much faith expenditure is to found your pre - national college empire in such a way that it buffers and stakes off a large amount of land from adjacent civ expansion, then pump out 3-4 settlers and a missionary to convert two of those new cities right away, that will create 18 pressure to convert the other new expansions within 5 turns after you settle them. The result is you'll have 30 pressure on all your cities and 18-24 pressure on nearby civs and city states. The ONLY way to passively spread a religion quickly is to have a lot of low population cities within 3-4 tiles of one another, 6-8 cities by turn 100.