Introducing a Fantastic Religion to the Players

[Vahnavoi]

Whenever I've run a game in the past I've always used a real world religion (i.e. the Greek pantheon). I worry that introducing something I made up would not give the players enough context to make reasonable decisions.

Am I being silly?

Depends. Are player characters coming to this religion as relative outsiders, or are they supposed to have inside perspective? The former is easier to work through than the latter.

With player characters coming to this religion as outsiders, the players' lack of context and knowledge is the same as their characters'. You can then introduce this religion to them, one important bit at a time. For example, an important holiday may be coming up. The players (and hence their characters) will observe the preparations. They will, reasonably, have questions, and, equally reasonably, there will be locals to answer them. They can then use the answers they just heard to form opinions and choose whether to participate.

If you want them to have an insider's perspective, you have to prepare a primer for it, the exact same sort of stuff real religions produce to spread awareness or gain converts. In practice, this entails making a setting book or booklet for them to read, or running a specific game scenario where they play through being initiated into this religion. How long or intense, depends on degree of devoutness you want them to display. Are your players supposed to play laity or clergy? Orthodox or heretic? How much leeway do you want to give your players for making up stuff as they go?

I've done this various ways. Often, the amount of information is too much for a player to absorb in one sitting, so there won't be much pay-off until many sessions later. But every once in a while, a player takes the ball and runs with it, becoming a memorable doomsday preacher or cult leader, or taking a principled stand against heresy, blasphemy etc. immorality that is happening in the game world.