Religious ideas help you to convert people to your religion more effective, faster and cheaper by giving you missionary strength +3%, one more missionary and missionary maintenance cost -50%, it also grants you with CB Religious war against countries with different religion and in the end, you get culture conversion cost -25%.
In the other hand, Humanist ideas help you to deal with rebellion issues in your country like the reduction of years of separatism by 10 years and national unrest -2 as well as it boosts your tolerance upon the heretic faiths and heathens by 2% and in the end, it grants with -10 idea cost.
So no, it does not stop the spread of other religions YOU are the one who accepts them or convert them