I don't know about the devil specifically, but I think the concept of hell makes a lot of sense and had a lot to do with the spread of religious myths, including Christianity. In a time when life was nasty, brutish, and short, where thieves stole your property and justice was scarce, where might made right, where so many poor people and minorities were oppressed, and where there was often little reward for goodness, a belief system that said all those bad people were going to get their comeuppance in the next life was incredibly attractive. You could be a jerk now, but you would be punished for your wickedness later. Rich people might be rich now, but would find it incredibly difficult to get into heaven later. Etc.

Hell was part of what made the myth so attractive. It wasn't just that you would live a better life. It's that all your enemies would get theirs. That all the justice you were denied in this world would be meted out in the next.

The problem lies in the definition. Jesus lived in a Jewish culture. Healing, plucking grain, etc. on the Sabbath was considered sin. Having contact with the unclean was sinful. Claiming to be the son of God... well sin enough to die. So did he sin? Depends on who you ask. By the OT definition of sin, most definitely. But the Christian reframed and redefined sin I suppose. So if breaking the law in the Hebrew Scriptures isn't sin, what is?

The answer is given in Heb 4:15; besides, Yeshua gave a clear statement that the Tanakh ~ "Law and the Prophets" will not be abolished in Mt 5:17.

Ha-satan exists, as Yeshua and biblical literature testify.

I am not a Christian but a Zen Buddhist. There is not a single religion in this world that does not believe in "devils"* or a devil.
As I have studied religions for the past 10 years. I have found out that generally speaking the biggest believers in Satan are the New Atheist. As the doctrine of Lucifer is common knowledge in the Hindu and Buddhist world, most of the new age scholars such as Blavatsky and Crowley took their ideas from there and not form the Bible.
That being said, any respected philosopher will tell you that the New Atheist people are post-modernist. As they preach materialism, they preach the doctrine of Lucifer which is "do as you please is the law".
As a part of a mind control study group we have found that authentic Atheism is almost taken over by the new age satanic Atheism called New Atheism.
So that been said, the question is not how many Christians believe in The Devil, but how many so called ''none believers" worship the doctrine of Lucifer without even knowing it.
To be honest, the level of understanding the concept of "the devil" is not very developed in Christianity as it is in Buddhism.
The problem lays in the lack of knowledge and education on this subject. A Buddhist type of individualism has nothing to do with the new age individualism related to Satanism. But we the western people can't understand the concept of the void. For us all that is seen is positive and all that can not be tested in experiments is negative. As we strive to make a neo-positive view of the world, we are losing our deductive reasoning and we fall victims of the cult to Satan (the devil).
I am really concerned that we as beings capable of dwelling in the realm of non-ego existence are forced into the materialistic hell of the EGO a place where ''the devil'' as the giver of the universal knowledge of good and evil ( the snake in the garden) is our only God and savior.
My will not understand me and yes many will never be good as Phil Ivey or Daniel Negreanu being just a game theory, math poker players who have problems look outside of the box.
On that note I have a question for you. Why is that Christians, who are keepers of high moral standards ( viewed from my Zen Buddhist stand point) are becoming victims of common sense and a self validating state of mind (ego) that is immune to its own falsehood?
What ever, I think that, the question is not how many Christians believe in the Devil but how many "non religious" people are worshiping the doctrine of Lucifer ( the Devil ) a doctrine that is found and dismissed, by 90%+ of all known world religions.
As most world religions see the Devil as a set of rules to gain material power over the world, it is almost save to say that the Devil is not ''who'' but ''what''.

Bodhidharma: Even if a buddha or bodhisattva should suddenly appear before you, there's no need for reverence. This mind of ours is empty and contains no such form. Those who hold onto appearances are devils. They fall from the Path. Why worship illusions born of the mind? . . . The basic nature of a buddha has no such form. Keep this in mind, even if something unusual should appear. Don't embrace it, and don't fear it, and don't doubt that your mind is basically pure. . . . Also at the appearance of spirits, demons, or divine beings, conceive neither respect nor fear. Your mind is basically empty. All appearances are illusions. Don't hold on to appearances. If you envision a buddha, a dharma, or a bodhisattva and conceive respect for them, you relegate yourself to the realm of mortals. If you seek direct understanding, don't hold on to any appearance whatsoever, and you'll succeed. . . . The sutras say, "That which is free of all form is the buddha." Disciple: But why shouldn't we worship buddhas and bodhisattvas? Bodhidharma: Devils and demons possess the power of manifestation. They can create the appearance of bodhisattvas in all sorts of guises. But they're false. None of them are buddhas. The buddha is your own mind. Don't misdirect your worship.

Bodhidharma (c. 440 AD - 528 AD)

red_Eyes:

Nowadays, I suspect many fundamentalist Christians believe in the devil. They talk about him all the time.

In the past, the devil was a very significant part of Christian folklore. Dante and Milton and Bosch are famous exemplars of this from the realms of art and literature.

Fobbing this off as some issue involving atheists and agnostics is incredibly intellectually dishonest. The devil is very much a character in the Christian folklore, and to some believers is an integral part of the theology.

Lucifer is inside of me, he speaks to me. He says "you're not a man, you are the Son of Man, and more than that, the Son of God, and more than that... God.

Sound about right. This is the kind of thing the devil would whisper.

Who believe in god should also believe in the devil. Its both equally ridiculous.

-- Saint Thomas Aquinas