Gospel of Luke

The Gospel of Luke was Luke the physician's account of the life of Jesus Christ. Its ninth chapter was once used by Bernice Summerfield to advance a theory about the identity of Jesus himself. Verses eighteen and nineteen of that chapter said, according to the King James Version that she was reading from:
 * And it came to pass as he was alone praying, his disciples were with hi: and he asked them, saying, Whom say the people that I am? They answering said, John the Baptist; but some say Elias; and others say, that one of the old prophets is risen again.

From this, she deduced that Jesus was John the Baptist. When pressed by Marillian to justify this unusual reading, she simply said, "It's as good a theory as any other". (PROSE: The Sword of Forever)

Behind the scenes
In truth, The Sword of Forever does not explicitly connect the book of Luke to Luke the physician. However, Byzantium! does mark out Luke as one of the gospel writers, so ascribing Luke the physician as the writer of Luke is essentially admitted by DWU texts.