Christianity makes a big thematic appearance in the trilogy in the fact that Luke believes that redemption is possible with his father. As for whether you can forgive Vader for his previous sins, that's relative based on one's personal moral beliefs. However, I think he's not comparable to Hitler, because he himself is not the evil orchestrator of the story. That is Palpatine. Vader is the epitome of how good intentions pave the way to hell, and he was manipulated by Palpatine. However, if you follow the Anakin/Jesus analogy (thats a WHOLE other debate, has many flaws, but still works in a lot of ways), he was tempted by the devil (a breif temptation in the Bible for Jesus in teh desert, but much more so in Star Wars, since Anaking actually BECAME Vader!), but ultimately had faith and believed in redemption and resisted the devil. This holds true, and is why Vader in my mind can be forgiven for his sins, while someone like Palpatine can't.