Which literary techniques does the author use to explore the theme of redemption in ‘A Christmas Carol’?

Redemption is a huge theme in A Christmas Carol, the entire purpose of the ghosts' visits is to give Scrooge a second chance and encourage him to make the most of his second chance; to redeem himself. The extended metaphor of the ghosts which represent self-reflection and morality is used to highlight the importance of redemption to Scrooge. 

Another extended metaphor is fire which links some characters to warmth and light, in contrast to the dark pathetic fallacy used to reveal Scrooge's 'cold' character. From Stave One where Scrooge is cold and refuses to use any more coal than necessary, to Stave Five where he tells Bob, “Make up the fires, and buy another coal-scuttle before you dot another ‘i’, Bob Cratchit!". The contrast of dark, smog, shadow, coldness, vs, light, fire, illumination, warmth and the final resolution of Scrooge choosing light makes this novella a clear allegory for redemption.