You may spot St Chritophe Cathedral from a distance before you actually find this grand religious building. Pop inside if it's open or just appreciate the facade from outside.
This principal church is free to visit, which is good news if you're vacationing on a budget. St Chritophe Cathedral is located in Belfort. When you're finished, Lion of Belfort, Belfort Citadel and Belfort Cathedral are close by and also worth a look.