Greetings Gold and Silver Level Templar Knights! You’re not going to believe me but that church in the picture was once a Roman bathhouse. Let me try and convince you!
Between 298CE and 306CE, the emperor Diocletian built a huge bath complex near what is now Termini train station. He was the last pagan emperor before Constantine converted to Christianity and unleashed a persecution to try and stem the upstart faith. He also constructed this gift to the people of Rome to keep them nice and clean.
Much of it was destroyed when the Roman Empire in the west collapsed but some of it endured in the Middle Ages as a fairly intact ruin. It was Michaelangelo who began work on transforming a big chunk of the old baths into a massive church - with the approval of the pope. The official reason from the Vatican was that Christian slaves had been used by Diocletian to build the bathhouse and their labours - and martyrdom - should be commemorated. I respectfully suggest there is no evidence whatsoever for this. The result was a roofed structure dedicated to Santa Maria degli Angeli e dei Martiri.
In the early 20th century, it was decided to remove a baroque entrance and show off the original brickwork from the baths, which some believe obscures the church by making it look like a ruin again. I quite like it myself. See below.