Which Pope would that be? Wikipedia only mentions there was some sort of 8th Century controversy, but what little survives doesn't touch on the shape of the earth: https://en.wikipedia.org/wiki/Flat_Earth#Early_Middle_Ages Hard to imagine this wouldn't be mentioned. I also noticed the reviews and reception of A World Lit Only by Fire are scathing, and if what they cite is correct, appropriately so.
It's my understanding its shape as a sphere was well accepted for a very long time, e.g. Wikipedia, citing a 1995 German PhD dissertation says "A recent study of medieval concepts of the sphericity of the Earth noted that 'since the eighth century, no cosmographer worthy of note has called into question the sphericity of the Earth.'"
See further https://en.wikipedia.org/wiki/Myth_of_the_Flat_Earth or the book I just found, Inventing the Flat Earth: Columbus and Modern Historians (http://www.amazon.com/Inventing-Flat-Earth-Columbus-Historia...), about how modern era "historians" created out of whole cloth the idea that it was thought to be flat back then. See the latter Wikipedia article over the issue with Columbus, which had to do with his incorrect estimation of the earth's circumference. If not for the convenient location of "the new world", he and his crews would have perished since he had the distance to the Far East off by a factor of 4. While his contemporaries weren't exactly sure of the circumference, they knew his voyage as envisioned would have ended in ruin.
It's my understanding its shape as a sphere was well accepted for a very long time, e.g. Wikipedia, citing a 1995 German PhD dissertation says "A recent study of medieval concepts of the sphericity of the Earth noted that 'since the eighth century, no cosmographer worthy of note has called into question the sphericity of the Earth.'"
See further https://en.wikipedia.org/wiki/Myth_of_the_Flat_Earth or the book I just found, Inventing the Flat Earth: Columbus and Modern Historians (http://www.amazon.com/Inventing-Flat-Earth-Columbus-Historia...), about how modern era "historians" created out of whole cloth the idea that it was thought to be flat back then. See the latter Wikipedia article over the issue with Columbus, which had to do with his incorrect estimation of the earth's circumference. If not for the convenient location of "the new world", he and his crews would have perished since he had the distance to the Far East off by a factor of 4. While his contemporaries weren't exactly sure of the circumference, they knew his voyage as envisioned would have ended in ruin.