Sentence:  The tragic irony of the play is that the very belief in and concern with a <WSD>devil</WSD> who could be met in the woods and combatted with formulae set out in books was the very thing that prevented them from detecting the real devil when he came among them . 
Meaning of the word marked with <WSD> tags:  (Judeo-Christian and Islamic religions) chief spirit of evil and adversary of God; tempter of mankind; master of Hell