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A Great Person specialized in religion. Prior to the Industrial Era, automatically appears when a certain amount of Faith is accumulated. The amount of Faith required increases with each successive Prophet. Can be purchased in cities with Faith from the Industrial Era onward.
Great Prophets are in the center of the new Religion system. Great Prophets are earned as the result of collecting Faith, and are the only unit that can found or enhance a religion. Once you've accumulated enough Faith (at least 200 in a Standard speed game), you have a chance to spawn a Great Prophet. You first use them to Found and Enhance a Religion, and then you may choose either to Spread the religion (usable 4 times), or to create a Holy Site. You may only use Spread Religion after you found one; you also cannot found more than one Religion per game. You may create a Holy Site at any time, even before you have Founded a religion. All actions except Spread Religion expend the Prophet. Once a Prophet has been used to Spread Religion it can no longer create a Holy Site.
As with other Great People, the Prophet's main actions (besides founding/enhancing Religion) will provide an immediate benefit (Spread Religion to convert more cities to your Religion and use their influence), or a long-term benefit (create Holy Site for a boost in Faith). Remember that once you start spreading religion with a Great Prophet, it can't create a Holy Site or enhance a religion anymore, so judge well what will benefit you more at the current stage of the game! Generally, creating Holy Sites isn't as useful as creating most other Great Person tile Improvements, unless you've filled out the Piety tree or the Religion you founded has serious use for extra Faith. In such cases, or when your Religion isn't widely spread yet, you should use the excellent power of conversion of the Prophet to enlarge your religious network and/or convert City-States which have set particular conversion missions.
The Great Prophet, like the Missionary, is able to enter the territory of other civilizations even without an Open Borders agreement. Unlike the Missionary, however, he doesn't lose power while inside, and may move relatively freely (obeying the normal constraints of moving civilian units in another civilization's territory). This allows for aggressive use of Prophets to Spread your Religion to nations that resist it.
Aside from being able to spread religion more times than a Missionary, the Great Prophet is able to remove existing religions from a city when converting, despite having the same strength (1000). Sometimes it's good to use it instead of Missionaries, when you need to spread your religion to a particularly resistant city.
The Pathfinder (the Shoshone replacement for the Scout) is able to generate a Great Prophet when uncovering an Ancient Ruins after turn 20 (on Standard speed). If you have not yet founded a religion you will receive a large amount of Faith.
Finally, be aware that a Great Prophet purchased in a city will represent the religion of the civilization that purchased it, which may be different from the city's majority religion (unlike Missionaries which will represent the majority religion of the city). In this way, even a civilization which has lost its Holy City can continue to spread its religion until all of its cities are captured.
"Great Prophets" explore the boundaries of religious belief, conveying their unique insights to guide the moral and ethical decisions of a civilization. They found religions, spread the word of god, and bring faith to the people. They provide hope to the nation during even the direst of times.