What is the theme in Sir Gawain and the Green Knight, Pearl, and Sir Orfeo by J.R.R. Tolkien?

Sin, righteousness and penance are all themes in this book. All three pieces in the compilation have deeply Christian themes. This should be expected as all three pieces were penned during the High Middle Ages in Medieval England. The symbolism of the text is full of religious icons, and items that in reality represent theological ideas. For instance, the nick on Sir Gawain's neck and the silk belt he received from Morgan de Lay both represent Sir Gawain's shame, sin and moral imperfection.