Nicolas Steno was one of the founders of modern stratigraphy and geology, and also dabbled in anatomy. His defining contribution to science was the idea that sediment is deposited in layers -- the farther down you dig, the older the rocks are.
A Lutheran for most of his life, Steno converted to Catholicism in 1667, a couple years before publishing his last scientific work. In 1675, he became a priest. Two years later, Pope Innocent XI appointed him a bishop.
In a TED-Ed lesson that hailed Steno as, "the most groundbreaking scientist you've never heard of," educator Addison Anderson elegantly summed up the priest-scientist's contributions:
"Nicolas Steno helped evolve evolution, broke ground for geology, and showed how unbiased, empirical observation can cut across intellectual borders to deepen our perspective. His finest accomplishment, though, may be his maxim, casting the search for truth beyond our senses and our current understanding as the pursuit of the beauty of the as yet unknown."