Capture it before someone else does. Remember that 70% of 20 is more than 90% of 15. That, and multi-religion helps you get a leg up on the tech race when you can go peaceful. Seriously--I've played some games that I race for religions and build as many monestaries as possible--to get the biggest +10% research bonus each. That, and later on you can get a good deal of happiness. Having multiple religions is good--as long as you know which religion is dominant. Spreading them equally is fine--especially since an opportunity to use all those religions may soon arise. Even if barbs may get it soon--you have a fighting chance of keeping it as it is. Just send defenders ASAP--even if you drain your core (protected) cities of their defenders. Holy Cities are always valuable.