According to various characters in the series, Teito possesses a beautiful/clean soul despite his tragic past. According to Frau, Teito's light was able to purify Landkarte's darkness. Kal later tells Teito that this was what enabled Landkarte to help him realize his faults and save himself.
According to Kal in chapter 94, a section of Teito's soul (the source of his light) was sealed away along with Pandora's Box (Verloren's body). Kal hypothesizes that if Teito can escape Verloren's core, his light is powerful enough to purify even Verloren's darkness.