Religion is subtle, but can be very effective, it's like a wonder that gets more powerful the more you spread it. Tithe is possibly the most powerful belief, +1 gold for every 4 followers is a massive boost to your economy if your religion becomes the dominant one. The Just War belief is very useful for aggressive civs, just have a great prophet accompany your army, have him convert the city you're attacking, and you instantly get a 20% combat bonus. Religion doesn't turn the game on its head, but it gives you extra leverage to help you achieve your goals.
There are also diplomatic bonuses for spreading your religion to other empires, if it becomes the dominant religion in another civ then you get a healthy diplomatic boost with them that makes it easier to become friends and form alliances. But if you forcefully spread your religion (using prophets and missionaries) to civs that have already founded their own religion, they'll get annoyed, and if you ignore their requests to stop then they may attack you.