He spoke at Marburg in 1934 against National Socialism.
It probably got him acquitted at the Nuremberg War Crimes Trials.
Well, von Papen was clearly not guilty of the Nuremberg charges, as he lost position and power way before the war and even if he formally remained in Hitler's cabinet (like Schacht, who has a sinecure as "minister without portfolio" until his 1944 arrest), it's unlikely he'd be in any position of responsibility.
Von Papen was obviously guilty, from historical and political point, for Hitler's rise to power. In Nuremberg Goering commented Ribbentrop did indeed deserve to be hanged, but for his stupidity. If incredible political stupidity were among Nuremberg charges, Papen would hung too.