Firefighters had actually been called to the building to help an ill patient, when someone noticed the flames.
The congregation was holding their annual Oktoberfest in the parking lot when the fire started.
One firefighter was hospitalized for heat exhaustion, and three others were treated on the scene.
The church was built in 1865 and served as a set for the 2013 Tom Hanks-Jude Law film "Road to Perdition."
A fund has been set up to assist with the loss. Donations can be mailed or dropped off at the First Community Bank & Trust in Beecher and Peotone.
Donations can also be given online through Lutheran Church Charities.