"The finest trick of the devil is to persuade you that he does not exist." - Charles Baudelaire...

The quote is less religious than it sounds and serves more as a metaphor. The devil is supposed to "trick" or tempt people into committing evil. Some stories depict the devil as disguising himself as a "good" person often looking "angelic" and whispering temptations to them. Essentially, the devil misleads his victims to believe that what they are doing is not wrong and sometimes even perfectly justified. This ties in to Hachiman's pessimistic philosophy that people will always convince themselves that they are not wrong and instead everybody else is (similar to the way Sagami pushed the blame away from herself during the culture festival and onto others instead).

This quote seems very similar with this one :“The greatest trick the devil ever pulled was convincing the world he was evil.” by Edmund Carver
I'm not very good at interpreting things but.. I see this quote as a lesson and "convincing the world he was evil" we could as well read it like "I am the evil".Anyway , what I wanted to say is that this quote says that as humans we should take responsability for our deeds and if they are classified either as good or bad we should assume them without taking the blame on either God or Devil saying "God helped me" or "The Devil made me do it!".But that is just my opinion.