The devil is a biblical concept. It appears in the Bible as a name for the human tendency to sin, which is what brings temptation to the human mind. The devil is the thing that tempts us; we tempt ourselves by our own thoughts. Jesus, of course, was born a human being and shared our temptations:
"Then Jesus was led up by the Spirit into the wilderness to be tempted by the devil." (Matthew 4:1)
The process of temptation for Jesus was exactly the same as the process of temptation for everyone else:
"Since then we have a great high priest who has passed through the heavens, Jesus, the Son of God, let us hold fast our confession. For we do not have a high priest who is unable to sympathize with our weaknesses, but one who in every respect has been tempted as we are, yet without sin." (Hebrews 4:14,15)
This passage likens Jesus to the high priest who was the head of the worship of God in the Temple in Jerusalem and who acted as a mediator between God and mankind. After his crucifixion, resurrection and ascension to heaven, Jesus acts as our high priest. The passage reminds us that Jesus was tempted in exactly the same way that we are. He was tempted by the devil, but like us it was his own desires that tempted him. We can therefore conclude that the devil is the tendency towards sin that is present in our inner desires.
The difference between us and Jesus in this context is that Jesus successfully resisted temptation, and as a result remained without sin. We fall to temptation and become sinners.
One can define the devil as that thing which tempts us. The Bible assures us that we tempt ourselves, that temptation is a product of our humanity. As the thing that tempts us is the devil, and the thing that tempts us is a part of human nature, so we must conclude that the devil is a part of human nature.
This means that there is no external devil. Many religious traditions claim that there is a supernatural entity outside humanity which tempts people to do wicked things. Of course, no such being can exist. The Bible is very clear that sin produces death.
"For the wages of sin is death, but the free gift of God is eternal life in Christ Jesus our Lord." (Romans 6:23)
"Behold, all souls are mine; the soul of the father as well as the soul of the son is mine: the soul who sins shall die." (Ezekiel 18:4)
These verses don’t only include human beings; the idea of a “soul” in Ezekiel applies to any living thing. Any being which sins must also die. This means that there can be no immortal sinners. A supernatural devil would be exactly such a thing, immortal because it lives for ever and a sinner because it tempts people to oppose God, which is exactly what sin is.
As immortal sinners can’t exist, a separate supernatural devil can’t exist either. Such a being would have to die, just as human beings die.