Well ... let's first establish that Evil DOES Exist - something which even Science admits to, but cannot explain it's origin! Second of all, let's also establish that Jesus Christ was a REAL Person - which even militant Atheist, Richard Dawkins, admitted in a debate with John Lennox. Third ... to quote C.S. Lewis, "I am trying here to prevent anyone saying the really foolish thing that people often say about Him: I’m ready to accept Jesus as a great moral teacher, but I don’t accept his claim to be God. That is the one thing we must not say. A man who was merely a man and said the sort of things Jesus said would not be a great moral teacher. He would either be a lunatic — on the level with the man who says he is a poached egg — or else he would be the Devil of Hell. You must make your choice. Either this man was, and is, the Son of God, or else a madman or something worse. You can shut him up for a fool, you can spit at him and kill him as a demon or you can fall at his feet and call him Lord and God, but let us not come with any patronizing nonsense about his being a great human teacher. He has not left that open to us. He did not intend to."
Now ... if we establish that Jesus was a real person and was who He claimed to be, then there are 9 times in the Gospels where Jesus mentions Satan, He speaks to Satan - directly as a being - and has interaction with, within a 40 day period, while walking through the wilderness. Jesus also spoke directly with Demons, who even identified themselves.
Therefore ... based on those facts, it is quite obvious that Satan is a REAL being, unless of course, you don't believe in God or that Jesus was an equal with God, The Father (a claim He made on numerous occasions) and was just some nice fellow or a lunatic.