Religious victory is easy if you jump on it quick. You can usually snuff out a few of the competing religions if you watch carefully and convert their cities that have holy sites as soon as they create their religion.
A civ will officially follow your religion if more than half of their cities follow it. You win if every Civ follows your religion.
Converting a city is dependent on population, and larger cities are harder to flip. But the victory is only dependent on number of cities.
So it will require less resources spent on apostles to convert the smallest cities and ignore the big ones if you can.
Also, missionaries should really only be used to convert cities that do not already have a religion. Otherwise use Apostles.
Try to build the wonder that lets you choose your apostles promotions, and get the +20 combat promotion and the "eliminate 75% of other religion pressure" promotions.
Winning religious combats spreads religion very well, and the 75% thing is also very efficient.