The story
Red-Letter refers to New Testament verses and parts of verses printed in red ink, to indicate the words attributed to Jesus without the use of quotations; see “Red letter edition” on Wikipedia.
Etymology from Wiktionary, CC BY-SA 4.0.
Red-Letter Christian
Today it means: A member of a non-denominational movement within Christianity, especially evangelicalism, that emphasises political activism on issues of social justice about which Jesus has made pronouncements. Full definition →