Hundreds of thousands of marriage records from 19th century Catholic and Lutheran parishes in the Poznan (Posen) region of Prussia have been indexed by volunteers and are available for you to search at The Poznan Project website.
If you find your ancestor in the index, you then have the parish and can search the Family History Library Catalog at familysearch.org for additional information available on microfilm through your local Family History Center.
Check the status of available records indexed to date, and more are being added as additional parishes are completed. This is a database to re-check periodically. Check out www.polishroots.org and the Poland page on the FamilySearch wiki to learn more about Polish and Prussian records.