I spent a lot of time in Churches and places of worship in Europe. Here are the top five I think you need to see in order to study the different styles of architecture and magnificence of the people's devotion back then.1. Notre Dame
There is a reason it inspired poets, authors and artists. Le Notre Dame is magnificent and blows all the other churches out of the water. Between her beautiful gothic gargoyles and high pitched roof make her seem scary to the inside where the candles and the high vaults make her seem friendly and warm. She's breath taking and a must see for any european vacation
2. The Church of Sacred Blood, Bruges
The only black church I saw on my whole trip. The black and gold outside make this gothic church seem more scary then usual. It also has a lot of red accents on the inside, hence the church of sacred blood. It is the most active of the any of the churches in Bruges with 7 daily masses so be respectable but you can experience the true catholic religion in this one and hear it's beautiful gold gilded organ playing.
3. St. Nicholas, Ghent, Belgium
Known as the Rose of the Ghent, Snt. Nicholas is the largest church in Ghent and the 2nd largest building. 2nd to the Belfry fortress in the town square. St. Nicholas is the home of the famous painting, "The Adoration of the Lamb". Go hear to see the painting and learn about it's sordid history of being covered up; due to the homely and naked depictions of Adam and Eve, being stolen by the Nazis in WW2 and being restored thanks to the people of Ghent. It's a triptych so walk around it and see all of it's sides.
4. Notre Dame, Strasbourg
Another Notre Dame but very different in style of architecture and feeling. Le Notre Dame in Strasbourg has a lacy and fragile feeling to it that no other church had on my journey.
5. St. Catherine's, Brussels
St. Catherine's is the largest church in Brussels and the most uplifting. It had a lot of light to it with it's beautiful stained glass windows and it's white coloring on both the inside and the outside. St. Catherine's is also in close proximity to many important city sites in Brussels so stop by and see her; she'll appreciate it!
As you can see by this list that I tried to pick churches from different cities with completely different looks as well. You can see a lot of churches and never see the same type of church again. Even if you are not relgious from an artist perspective the dedication and beauty of old churches can be seen to even the most comfirmed atheist.