Often we wonder why evil is allowed to exist, what the ultimate end is in allowing the enemy to remain in existence at this time. What purpose could possibly be served by this? In exploring God's word, we find many reasons this is the case. While only He alone ultimately knows for sure, we are made privy to several clues:
 
He is the god of this world.
Ephesians 2:1-2,  "And you he made alive, when you were dead through the trespasses and sins in which you once walked, following the course of this world, following the prince of the power of the air, the spirit that is now at work in the sons of disobedience." 
1 John 5:19,  "We know that we are of God, and the whole world is in the power of the evil one.  
2 Corinthians 4:3-4,  And even if our gospel is veiled, it is veiled only to those who are perishing. In their case the god of this world has blinded the minds of the unbelievers, to keep them from seeing the light of the gospel of the glory of Christ, who is the likeness of God."  
  Now it has been often said that he has this status because he's the one that everyone worships. While it is true that to reject Christ is to choose Satan(whether one intends to do so or not), Luke reveals a more accurate and gut-punching explanation:
Luke 4:5-6,  "The devil led him(Jesus) up to a high place and showed him in an instant all the kingdoms of the world. And he said to him, 'I will give you all their authority and splendor, for it has been given to me, and I can give it to anyone I want to...' " 
  A statement such as this can come as a shock..."It has been given to me." But this is not intended to mean that the devil has literally taken over or assumed ownership of the world. He actually does not even have any dominion over the earth. That is still a human responsibility. He is referring to that fateful day at the beginning, in the garden, where man chose evil over good and the word of a snake over the counsel of the Living God. Like the immoral brother in I Cor. 5, we were handed over to Satan in order that through Christ's sacrifice our sinful nature may be destroyed and our spirits saved "on the day of the Lord."

He is sometimes used to fulfill God's purposes
Job 1:12  "The LORD said to Satan, "Very well, then, everything he has is in your hands, but on the man himself do not lay a finger." Then Satan went out from the presence of the LORD." 
Luke 22:31-32  "Simon, Simon, Satan has asked to sift you as wheat. But I have prayed for you, Simon, that your faith may not fail. And when you have turned back, strengthen your brothers."  
  Every test, trial, and temptation that is common to man brought against us by the enemy is ultimately allowed by God. In Job's case, it was to demonstrate to Satan his faith in Job's ability to keep his integrity in the face of suffering. God works in concert with our obedience (or perhaps the lack of it) to bring glory to Himself and move world events along in accordance with His plans.
  By the time of the Luke verses listed, Jesus had already renamed Simon as Peter. But here, Jesus reverts to his old name to remind him from whence he came, possibly as motivation to exercise his faith and resist the evil that was to come. Soon after this conversation is when Peter denied Jesus three times. Paul reminded his hearers and readers many times of the sinful nature they originated from. Classical philospher George Santayana summed this strategy up brilliantly: "Those who ignore history are doomed to repeat it." 
John 13 27-28,  "As soon as Judas took the bread, Satan entered into him. 'What you are about to do, do quickly,' Jesus told him, but no one at the meal understood why Jesus said this to him." 
  The gospel is often mistakenly portrayed as the devil delivering a knockout punch and rife with arrogance in the belief that he defeated Jesus on the cross. This verse reminds us that God was controlling his movements all along, and he knew it.

II Cor. 12:7-10 To keep me from becoming conceited because of these surpassingly great revelations, there was given me a thorn in my flesh, a messenger of Satan, to torment me. Three times I pleaded with the Lord to take it away from me. But he said to me, "My grace is sufficient for you, for my power is made perfect in weakness." Therefore I will boast all the more gladly about my weaknesses, so that Christ's power may rest on me. That is why, for Christ's sake, I delight in weaknesses, in insults, in hardships, in persecutions, in difficulties. For when I am weak, then I am strong.

  We are not told specifically what the actual thorn in the flesh is. Some scholars say it was an eye disease he may have suffered from in the latter part of his life, while others maintain the thorn is spiritual. Whatever it was, it was obviously quite troubling to he who was arguably one of the greatest apostles. It proved to be one of many effective tools God used to bring him to the point where he could say, "...I have learned in all things to be content...whatever the circumstances." (Phil. 4:11)

I Cor. 5:1-5 "It is actually reported that there is sexual immorality among you, and of a kind that does not occur even among pagans: A man has his father's wife. And you are proud! Shouldn't you rather have been filled with grief and have put out of your fellowship the man who did this? Even though I am not physically present, I am with you in spirit. And I have already passed judgment on the one who did this, just as if I were present. When you are assembled in the name of our Lord Jesus and I am with you in spirit, and the power of our Lord Jesus is present, hand this man over to Satan, so that the sinful nature may be destroyed and his spirit saved on the day of the Lord."

I Tim. 1:20, "Among them are Hymenaeus and Alexander, whom I have handed over to Satan to be taught not to blaspheme."
  Looking carefully at the two passages above, we see that Paul is not intending to literally consign these people to eternal hellfire. What he is doing here is more of an excommunication of sorts, expelling the wayward believers from Christian fellowship. This is at times necessary to preserve the integrity of the individual congregation, or the body of Christ as a whole if the incidence of sin is on a wide enough scale ("If your right eye causes you to sin, gouge it out and throw it away. It is better for you to lose one part of your body than for your whole body to be thrown into hell." - Matt. 5:29). It's difficult to tell if Paul meant this action to be a permanent measure, but clearly his hope was that those cast out would see their need for repentance and refinement, resulting in restoration to God's family.
 
His defeat is assured...but we are not the ones to deliver it.
 John 16:11 "...the prince of this world now stands condemned."
  This is akin to saying that the devil is on death row...a dead man walking. His fate has been sealed. So why does he still roam free if that is the case? The reason for this, possibly, is that for God to bind him now would remove the freedom of choice that every person has to accept or reject salvation. It also would preclude God's use of him in the scope of His will, as explored above.

Rev. 20: 1-3, 7-8 "And I saw an angel coming down out of heaven, having the key to the Abyss and holding in his hand a great chain. He seized the dragon, that ancient serpent, who is the devil, or Satan, and bound him for a thousand years. He threw him into the Abyss, and locked and sealed it over him, to keep him from deceiving the nations anymore until the thousand years were ended. After that, he must be set free for a short time...When the thousand years are over, Satan will be released from his prison and will go out to deceive the nations in the four corners of the earth--Gog and Magog--to gather them for battle. In number they are like the sand on the seashore."

  Believers are quick to "bind Satan" in their prayers. They fantasize him shaking in his boots at these proclamations. But we are shown here that any binding that is to be done will actually occur in the end times. So what hope do we as believers have of victory in this day and age? What tools do we have to prevent his infiltration of our lives? A few answers to this will be explored in the next and last installment of this series.
 
Coming Up Next - Part IV: Disengaging the Enemy