This is one of the first Wildcards available in the game, and maybe the most important one, at least for players that intend to Found a Religion. It will give you +2 Great Prophet Points per turn, even without a Holy Site; or if you dutifully construct it and its first building (the Shrine) - 4 GPPs total, plus any eventual bonuses. Although this may not be enough to earn the first couple of Great Prophets (considering how ridiculously buffed the AI is in the beginning of the game on higher difficulties), it will ensure you get one soon afterwards.
Remember that in order to use this Card you'll need to develop Political Philosphy, as well as Mysticism (unless you're playing as Greece - then you will have a Wildcard slot already and won't need Political Philosophy). Now, Mysticism is not required for Political Philosophy, which means that you'll have to go our of the 'straight' way in order to get it. If you choose to do it before going for P. Ph., then you'll be able to slot Revelation immediately after you switch to the more advanced Government; otherwise you can do it after that, and use another Card in the Wildcard slot for a few turns - both approaches have their merits. In the first case you will get early access to the Oracle, if you wish to construct it, as well as an extra Envoy; in the second you will get a more advanced government a little sooner, and you'll get to use its benefits.
If you don't care about Religion, you shouldn't use Revelation at all. Go for a more generally useful card, such as Inspiration, or even use a non-Wildcard card in this slot to boost another aspect of your gameplay.
Civilopedia Entry Edit
Religiously speaking, revelation is the revealing of some form of truth or knowledge through direct communication with a deity or other supernatural entity through various means – burning bushes, graven tablets, voices from statues, drug-induced dreams, and the list continues. Virtually every prophet from Moses to Mohammed to Joseph Smith has claimed the revelation to them personally of holy writ. Whether confabulation or fact, there is no denying that divine revelations have shaped civilization throughout the millennia. Recent scientific studies by Stanley Koren have sought to understand the phenomenon through the use of the “God helmet,” a device for stimulating the temporal lobes … perhaps soon everyone can have revelations.