There is no key for the hatch beside the church in the village.
When you kick the seekers out of the Black Boar inn, if you tell Louis it wasn't a big deal, things just got out of hand, you can talk to them in the barracks. They will be grateful and one will tell you about their commander's loot stash under the church, and that he often forgets to lock the hatch (coincidentally, as soon as you know this the hatch will be unlocked). Whether you loot the place or not, you can rat them out to their commander for revealing his secret.
If you get the seekers sent on a dangerous mission as punishment, you may not be able to get in (you will encounter them later fighting a troll, near Lovis' tower; if you are fast enough you can talk to them and get the hatch opened).
