The Bible regards Satan as a real being. He isn't a myth, a feeling, generic "evil". He is an actual angel who fell from his first estate (Luke 10:18). He is real.
In fact, the Bible has a lot to say about Satan. He was in the garden of Eden in the form of a serpent (Genesis 3:1ff) 1. He is called "the god of this world" (2 Corinthians 4:4), "the prince of the power of the air" (Ephesians 2:2), and other terms indicating a certain measure of power. He brought about Jesus's betrayal (John 13:27) and has "dominion" (Acts 26:18). It says in Hebrews 2:14 that he had "the power of death". Interestingly, while he has all this power, even to hinder God's people (1 Thessalonians 2:18), to disguise himself as an "angel of light" (2 Corinthians 11:14), even seeking to devour believers (1 Peter 5:8), it would appear that he is limited by God. Evidence, for instance, the events of Job. In the first two chapters of the book of Job, Satan was required to present himself to God (Job 1:6; 2:1). Indeed, nothing that Satan accomplished in Job's life was without permission and restriction from God. So Satan is powerful, but limited. In fact, on more than one occasion in Scripture Satan and his demons were used by God to accomplish what He intended. For instance, God sent an evil spirit to cause friction between Abimelech and the men of Shechem (Judges 9:23), an evil spirit to torment King Saul (1 Samuel 16:14), a deceiving spirit to entice Ahab to his death (1 Kings 22:23), and Satan himself to discipline believers (1 Corinthians 5:5; 1 Timothy 1:20).
So, if Satan can be used by God and Satan can disguise himself as an angel of light and Satan can even tempt the Son of God, how do you know when he's at work and not God? Jesus offers a fascinating insight into this question. Remember the famous "Who do you say I am?" question He asked of His disciples? In that event, Peter boldly declares, "You are the Christ, the Son of the living God" (Matthew 16:16). Good stuff, Peter. Perfect. So perfect, in fact, that Jesus says, "Blessed are you, Simon Barjona, because flesh and blood did not reveal this to you, but My Father who is in heaven" (Matthew 16:17). And then Jesus went on to tell them that He was going to die. Wait ... what? No, Peter didn't take that well, either (Matthew 16:22). I find Jesus's response to Peter very revealing. The Scriptures say that Jesus rebuked Peter by saying, "Get behind Me, Satan!" (Matthew 16:23). Interesting, isn't it? It was Peter who was talking and Peter who was rebuked, but it was Satan who caused it. So, what was Jesus's clue? How did He know it wasn't just misguided talk, but was actually the work of Satan? He doesn't leave us to guess.
"Get behind Me, Satan! You are a stumbling block to Me; for you are not setting your mind on God's interests, but man's" (Matthew 16:23).
And there you have it. How do you know if your thinking and planning and feelings are in line with God or with Satan? How do you know when you're being deceived by the god of this world or lining up your mind with God? What is the key piece of evidence that told Jesus that Peter wasn't merely mistaken, but actually working from Satan's perspective? "You are not setting your mind on God's interests, but man's."
Without a doubt in my years of experience with finding errors in my own thinking as well as addressing the errors that others have raised, especially among those who call themselves Christians (whether or not they actually are), is this very measure. The consistent, ongoing, ever-present error you will find is when you set your mind on man's interests and not God's interests. Indeed, this answers a vast array of problems offered by unbelieving skeptics and even faithful but confused Christians. It answers the problem of evil, questions about the justice of God in destroying the Amalekites, questions about the fairness of the role of women versus the role of men, questions about who gets saved and who doesn't, questions about the reality of Hell, and on and on and on. All of these and more are a problem when viewed from the interest of Man, but not the slightest difficulty when viewed from God's interests. If you would like to determine where the most likely place your thinking will go wrong, look no further. Ask yourself, "Am I most concerned in this particular issue with the interests of God or with the interests of Man?" I think you'll find it clears up a lot of questions. And remember the source of that error: Satan. An easy and very common mistake to make. Let's see if we can avoid it.
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1There are those who would actually argue that it was "a serpent" and nothing more in the Garden of Eden. I would argue that it was Satan, and that without question. Revelation on more than one occasion refers to Satan as a serpent (Revelation 12:9; 20:2). He is called the "father of lies" (John 8:44), which is what the serpent in the garden was using. And God says, "You were in Eden, the garden of God" (Ezekiel 28:13). Beyond all this (and more), this is one of those things on which the Church in its 2,000 years of history has always agreed. I don't think there can be much doubt here that Satan was represented in the Garden of Eden as the serpent that tempted Eve.