The claim is true as a broad historical characterization. UK Parliament describes the Civil War as involving religious conflict and, more specifically, tensions around the Protestant English Church and Catholicism, while English Heritage says the wars were fought between Charles I and Parliament and divided society along political and religious lines. That makes "Christian against Christian" an accurate shorthand for a conflict within Christianity, even though it simplifies the war’s broader political causes. Sources: English Heritage, "The English Civil Wars" (https://www.english-heritage.org.uk/learn/histories/the-english-civil-wars-history-and-stories/the-english-civil-wars/); UK Parliament, "Great rebellion, English Revolution or War of Religion?" (https://www.parliament.uk/about/living-heritage/evolutionofparliament/parliamentaryauthority/civilwar/overview/great-rebellion/); UK Parliament, "Catholics and Protestants" (https://www.parliament.uk/about/living-heritage/evolutionofparliament/parliamentaryauthority/revolution/overview/catholicsprotestants/).