The etymological meaning of Lucifer is “bearer of light,” making Luciferians consequently the bearers of light and thus of the truth.
Also, last I checked, Protestantism has its origins in the 16th century, before which time, Christians were almost universally Catholic or Orthodox, and it is they who invented the rule that all true doctrine must come from the Bible (note that that rule isn’t anywhere in the Bible…). Talk about changing the rules, huh?