There are a couple of options for reviving an endangered (or “dead”) religion in Civ 6, if you don’t have a majority religion in your last city:
- After declaring war, killing your opponent’s religious units with your military units automatically reduces your opponent’s religious pressure (which may be enough to push one or more of your cities back into the light). Use cavalry units for quicker movement, scouts work too.
- If not built yet, the Mahabodhi Temple grants two apostles of your founded religion – even if it doesn’t exist anymore. But you have to build it in the city with no religion. Use Apostles to “revive” a city with a holy site and start spamming more religious units (inquisitors especially).
You can chop it with Magnus pretty fast, because it is relatively “cheap”. - If Vatican City is in the game, becoming its suzerain will allow you to “spread” your founded religion every time you use a Great Person.
- This last one is pretty late, not sure when exactly you would use it.
The “Rock God” promotion for your rock band converts an opponent city to your founded religion. As a last resort, perform the concert in one of their cities with a holy site, then after it converts, declare war and conquer the city, and spam religious units.
The point to all of this is that once you get a city back under your religion, you spam units ASAP. That, or check out the tenets and beliefs of the enemy religion… it might be worth keeping for yourself!