Fun fact: before the Protestant Reformation, the words Catholic and Christian were essentially synonyms, and the capitalization to Catholic wasn't added until they needed a way to differentiate themselves from the other branches. Catholic comes from Old French catholique, which comes from catholicus, which comes from Ancient Greek katholikos. Going from katholikos to present day, the definition was consistently the same, but that's where we experience a split. Katholikos also meant "universal" in Ancient Greek; it sort of makes sense, when you consider the inclusivity of everything in their belief system. We can break this down into the words kata, meaning "about", and holos, meaning "whole" (so according, to the Greeks, a universe is "about the whole"). Kata might be from Proto-Indo-European kom, meaning "beside", and holos is reconstructed as deriving from a PIE root sounding like solh and meaning "whole" as well.
Adam Aleksic, a freshman studying linguistics at Harvard University, has been described as the internet's sixth most famous etymologist. He has disturbing interests in words, vexillology, geography, board games, limericks, and law, and he loves writing about himself in the third person.
The Etymology Nerd