Question: I have a friend who is very strong on the point that there is really no "literal" devil. How necessary is this belief to being a Christian?
Response: There is indeed a literal devil (for refs and info see Part 1 of The Satanic Rebellion: Background to the Tribulation). Now there are many people who profess Christ who don't believe a lot of things that are clearly taught in the Bible, and I dare say that most Christians have a hard time with at least something in the Bible. We are here to grow in faith and grow closer to God - something only possible through intense study of and attention to His Word. We have to put aside all our doubts and let the scripture be our guide.
It is possible "not to believe" a point of scriptural truth (such as the one you ask about) and "still be a Christian", but, it is well to consider that ALL truth is there in the Bible for a reason. Everything we reject merely hardens our heart more against the truth as a whole; everything we fail to believe in the Bible merely makes us vulnerable to the attacks of the evil one. We grow by learning, and by believing what we learn, and by applying what we believe. To reject scriptural principles out of hand is like deliberately shooting oneself in the foot. Obviously, one can take only so many such "shots" before the consequences are disastrous. I can't (and wouldn't wish) to judge your friend, but my overall response to you and to him and to all who show reservations about believing the Bible is "what, then DO you believe?" We know about Jesus FROM THE BIBLE. Who are we to pick and choose what is right in the Bible and what is wrong? It is all right, and we can only stay spiritually safe by believing it all; we can only grow spiritually and come to serve our Master most effectively by learning everything the Bible has to teach us. If the devil isn't literal, who or what in the Bible is and who is going to tell us that and on what basis?
Please see the links:
Read your Bible: Protection against Cults
The Cross, Sin, and the Devil in God's Plan.
The Satanic Rebellion
Hope this helps you in some way.
Yours in Christ,