Marvin Meyer provides an English translation of an early Christian text that describes Cain as "the sun."
Marvin Meyer, "The secret book of John (The Apocryphon of John)," in The Nag Hammadi Scriptures: The International Edition, ed. Marvin Meyer (New York City, NY: HarperOne, 2007), 115
The fifth is Adonaios, who is called Sabaoth.
The sixth is Cain, whom generations of people call the sun.
The seventh is Abel.