Stadthagen is part of the Evangelical-Lutheran Church of Schaumburg-Lippe, which is no member of this society, Kirchenbuchportal GmbH (Archion), who operates this platform.
That said you will not find any digitized church records/registers for Stadthagen here. (As an aside: Stadthagen church registers are so far no where online accessible!)
Even FamilySearch does not have any church records/registers for Stadthagen.
You may consider to contact the following State Archive in Lower Saxony holding microfilmed church registers for Stadthagen. Please be aware that charges will apply for their services, so don't forget to ask.
Niedersaechsisches Landesarchiv Bueckeburg
Schloßplatz 2
Bueckeburg
Niedersachsen
31675
Germany
Email: Bueckeburg@nla.niedersachsen.de
Good luck.
