The Devil is the supreme enemy of God in Christian mythology. But where did this idea come from?

Not from Judaism. Separate concepts in the Jewish tradition are reinterpreted and combined in order to fit into the Christian concept of an arch-enemy working against God.

Ba’al
Ba’al is a rival deity that Yahweh fights and defeats in Jewish mythology. This is the result of the Yahweh worshiping Jews defeating the Ba’al worshiping Jews.

Lucifer
The concept of Lucifer is derived through the misinterpretation of Isaiah 14:9-17. This passage refers to the King of Babylon, a man and not an angle, who was said to seek greatness above God but whose pride was followed by a fall. “Morning Star”—which was translated as
Lucifer—was a title given to the King of Babylon in accordance with the belief that he was of divine parentage in association with Venus (the morning star).

In Ezekiel 28:12-19 we again find a description that is taken as evidence for a fallen angel but which is originally intended to describe the fall of another earthly king as a result of pride, in this case the King of Tyre.

Azazel
Azazel is described in two places, not necessarily being the same entity in both. In Leviticus it is a type of demon in the wilderness, to whom a goat is released in addition to the one sacrificed to the lord. In the book of Enoch, Azazel is a leader of a group of angels known as the watchers. After coming to earth the watchers fall in love with human women and bear children by them called the Nephilim, who were a race of giants. Azazel also taught men about weapons and women about cosmetics. The fact that humans had been given divine children was one of the reasons God created the flood, so that his spirit would not remain among them. The flood, of course, is also related to the fact that Yhw(h) was originally the god of chaos, destruction, and the stormy sea in the Levantine pantheon (see YHWH).

The Serpent in the Garden of Eden
This was originally taken to be an ordinary serpent (though a speaking one, as is common in fables) and was only later associated with the concept of Lucifer forged in Christianity.

Satan
The term ha-Satan, or the-Adversary, comes from the title (though not the name) assigned to an angel in Yahweh’s court. He was a prosecutor for God against human individuals, with God as the Judge. Ha-Satan could not act without God’s permission and did not cause people to sin, though God may have ha-Satan test a person, as in the case of Job. Ha-Satan’s primary role was to record and point out the sins people committed and argue against them in the holy court in accordance with God’s will.

In none of these cases are we given the Devil in the Christian sense—the once favored angel of God that rebelled, was cast from heaven into hell, and now seeks to tempt/trick/torture human beings in opposition to God’s will. Far from Yahweh having created a being that became his adversary, in a number of places, Isaiah 45:7 being one example, Yahweh proclaims that he himself makes both good and evil.

But if the Devil doesn’t come from Judaism, what is its source? Once again, Pagan influence. The Zoroastrians believed that there were two gods, one good and one evil, in constant opposition and competition for human souls. In addition to this, Horus, with whom Jesus’s life shares 200 points of similarity, is said to have been taken from the desert of Amenta up a high mountain to be tempted by his arch-rival Sut (or Set). Later Jesus is said to have been taken from the desert in Palestine up a high mountain to be tempted by his arch-rival Satan. Both resist temptation.
Satan is an essential character for Christianity, possibly even more so than Jesus. Without the threat of Satan, Sin, and Hell, Jesus would serve no purpose. In order to convince people that they need Jesus to be connected to God, one first has to convince them that they are not already connected to God, and this has been the great goal of the missionaries. That this is ultimately a manipulative fear tactic in order to control people and gain institutional power should be obvious.

Arch Angel Lucifer
As a biproduct of Lucifers jealously of god, he was cast out of heaven and took 1/3 of all angels with him. These 1/3 angels were damned willingly and unfaithfully to god. Lucifer was holy but angels too can go on their own volition, thus becoming fallen angels but unlike humans, cannot be redeemed like human kind.

1. Prince of Darkness; King of Hell
2. Anti-Christ
3. The excuse for everything bad that happens in the world.

The excuse for everything bad you do in your life.

An authority figure who takes pleasure in tormenting his/her underlings.

George W. Bush - President Of the U.S. of A.

Deity that all highschool spanish teachers worship.

Devil(Satan, etc.)is the enemy.He stands for chaos as god stands for order, it represents rebbellion in opposition to god who represents oppression. In christian mythology Devil was an angel, damned for His disobedience