This Devil is not an atheist, not a Buddhist, nor is he a believer in any other religion. The only religion that this Devil believes in is the Christian religion. This Devil is a sinner, probably the worst sinner ever. However, Christians can be sinners, so that does not exclude him. QED.
While the devil does believe in God (obviously, he got thrown out of heaven by him!), this does not make him a Christian. A Christian is someone who believes in God, and believes that His Son Jesus Christ died on the cross to forgive people of their sins. A Christian does not only believe this in their head, but in their heart (kind of a vague term, but you get what I mean). While the devil knows this, he can't and won't accept forgiveness for committing treason against a Holy God. He tries to drag as many people down to hell with him as possible, which hardly sounds like the behavior of a saved and forgiven Christian (side note: the devil, contrary to popular belief, does not rule hell, rather he is trapped in it). Anyway, interesting opinion, not sure I've ever heard it asked before. Hope this helps others see the other side of the argument. :)
First of all, there is no redemption for the devil or any other fallen angel:
"And the angels which kept not their first estate, but left their own habitation, he hath reserved in everlasting chains under darkness unto the judgment of the great day." (Jude 1:6)
"Then shall he say also unto them on the left hand, Depart from me, ye cursed, into everlasting fire, prepared for the devil and his angels." (Matthew 25:41)
Belief is essential to becoming a Christian, but being a Christian consists in much more than simply belief. It is a spiritual rebirth:
"Therefore if any man be in Christ, he is a new creature: old things are passed away; behold, all things are become new." (2 Corinthians 5:17)
"Jesus answered, Verily, verily, I say unto thee, Except one be born of water and the Spirit, he cannot enter into the kingdom of God! That which is born of the flesh is flesh; and that which is born of the Spirit is spirit. Marvel not that I said unto thee, Ye must be born anew. The wind bloweth where it will, and thou hearest the voice thereof, but knowest not whence it cometh, and whither it goeth: so is every one that is born of the Spirit." (John 3:5-8)
If by "believe" you mean he knows God exists and that Jesus died on the cross for the sins of man, then yes he is a believer. But that doesn't make a Christian. He may know that everything God says is true, but that doesn't mean he accepts it. You see, I realized something not long ago. Doesn't it seem a little out of character for God to say "Believe in me or die."? Seems a bit harsh to me. But then I realized that we've been misinterpreting this all along. It's not about "fear me or die", but about his gift. He is giving us an option. The way I now interpret this is "I'm offering you an option. I gave you free will for a reason, and I'm not going to take it away now. I'm giving you something: the gift of life. The gift of salvation. It's your choice as to whether you accept it." You see, he's not sending us to hell for "disobeying his orders" by not believing. We are sending ourselves to hell by rejecting his gift of life. If we don't want to be saved (or in other words, refuse to believe that he can save us/refuse to believe that he exists at all), then he won't force the gift upon us.
In this context, the Devil is not a Christian, because a Christian is someone who believes that God can and will save them, and has accepted this gift. Satan has completely rejected this gift, therefore condemning himself to Hell. He may "believe" that God exists and know that everything he says is true, but he is not a true Christian because he has refused God's gift.
Although the devil believes that the gospel is true, he is not a Christian because he is not a follower of Christ. Simply assenting to the truth of Christianity does not make one a Christian. A person becomes a Christian when God changes their heart, resulting in them repenting of their sins and coming to Christ for salvation.
My default dictionary defines Christian as "a person who has received Christian baptism or is a believer in Jesus Christ and his teachings." The devil violates nearly all of these criteria. Even without the "person" criteria (which I'm not a big fan of anyway), a Christian necessarily believes in Jesus' teachings -- the Devil does not.