One explanation why church doors are often red is that it indicates sanctuary (in the sense of a place you can go when in trouble).

Another related explanation is that no one should ever have trouble figuring out how to get into a church.

This used to be a church - First Presbyterian, Detroit - but is now home to a seminary, the Ecumenical Theological Seminary, Detroit. It's a fledgling seminary, and the building is being fixed up little by little, but meanwhile inside, dedicated students and teachers are examining how to minister to people in a place like Detroit, where you can earn an advanced degree inside a building that looks like this. ETS is a great school - I audited several classes there - and specializes in evening classes for people who are answering the call to ministry at a later age, as a second or third or more career.

It's also a cool looking door, I think.

The "new" Roman Catholic cathedral in San Francisco, a.k.a. "St. Mary Maytag" or "Our Lady of Perpetual Agitation" - a photo of the whole church will explain those nicknames!

Pilgrim Church, Detroit

I always liked the image of Jesus brooding over a gas station. Now, of course, the fact that it's BP is even better. I was a passenger, but this was taken out of the window of a car driving by.

The church is the Shrine of the Little Flower, Royal Oak (Detroit suburb), Michigan. It's a Roman Catholic church, dedicated to Ste. Thérèse de Liseaux, a.k.a. the "Little Flower." They have a relic of her there, which is what makes it a shrine, I think.

And you're not the first person to think it's phallic.

This is a little church called Grace to Grace Church, located next to the Michigan Central Station, Detroit.

We're looking at it from behind. The rounded portion is known as an "apse".

Interior, ceiling/windows of the Roman Catholic cathedral in San Francisco, CA.

Originally an Irish Catholic church by the name of St. Agnes, this church later became predominately African-American (as is the city of Detroit, where this is) and was re-named Martyrs of Uganda Catholic Church. It used to run a school, but now isn't even a church anymore, just an empty building. The building itself is beautiful. I used to live nearby. It's about a block north of the Motown Museum, on Rosa Parks, across the street from where my profile picture was taken.

I don't know if there used to be stained glass here - often when a church is closed, its fixtures will be salvaged and/or sold off by the diocese, for use in other churches, or for other uses. You can see a chain hanging from the ceiling, which would have at one time held a light fixture that was either salvaged or stolen. [Edit: oops, no you can't; must have been in one of my other photos. Still, I thought the comment was worth leaving.]

Lots of empty Detroit buildings have been looted. I remember some years ago reading an article about how an upscale community in Chicago had the lions on its gates that were stolen from the Lee Plaza building in Detroit - a perfectly salvageable building, unless it gets further vandalized or looted. That's just one example.