The majority of the city-state's population needs to follow your religion, for it to become the dominate religion--self-explanatory. So if the city-state has a population of 13, your religion needs 7 followers. The amount of pressure that your religion is placing on the city-state represents the rate at which the population is being converted. If another religion is also applying pressure then depending entirely on which religion is applying more pressure you may never acquire the majority.