|Location||St. Hadwyn's Mission|
Brother Gamian is a follower of Mitharu. He lives as a monk in St. Hadwyn's mission in the very north of Glendara.
When the you first meet Brother Gamian, he asks if you worship Mitharu. You can lie to him and say you do (95% success with Persuasion 3), or you can say you don't, the third option is "I follow my own path".
You can buy shards and gems from him regardless of your answer, and his prices are no lower if he thinks you're a worshipper.
If you lie to him, he says you'll be welcome in all of Mitharu's homes, so maybe this affects other NPC's reaction towards you (unconfirmed).
"Are you a follower of Mitharu, here to worship?"