Under what conditions can I force a religion change in peace talks?

[Extreme Unction]

I got into a war with Sweden on behalf of my ally, Poland, who has a bug up her butt about Sweden in my game. But since I had the biggest army and the most prestige, I wound up as the leader of the alliance.

When time came to talk peace with Sweden, I demanded that she convert to Orthodoxy. And she did.

But I've been in a few wars since, and that hasn't been an option.

Do I only get the forced conversion if the country is Heretic rather than Heathen?

 

[Caban]

Yep.

Only way to do a force conversion for heathens is to literally conquer them, convert their capital province and release.

 

[Pewt]

Yep.

Only way to do a force conversion for heathens is to literally conquer them, convert their capital province and release.

If you release them via Create Vassals they will be released as your religion if they otherwise wouldn't be in your religious group.