Student Question--"Why Does God let Satan take over people's lives?"

Even though the answer is fundamentally the same, let me answer this question in two parts. First I'll address the believer in Christ, then a brief comment on unbelievers.

As for believers Satan is not able or allowed to take over a believer's life. Scripture teaches:

ESV 1 John 4:4 Little children, you are from God and have overcome them, for he who is in you is greater than he who is in the world.

As you read your New Testament you will see the reaction of demons when they come in contact with Jesus (...He who is in you!):

ESV Luke 8:28 When he saw Jesus, he cried out and fell down before him and said with a loud voice, "What have you to do with me, Jesus, Son of the Most High God? I beg you, do not torment me."

That said, Satan and his kind do have power to influence us from the outside. Notice when we are encouraged to put on the full armor of God:

ESV Ephesians 6:16 In all circumstances take up the shield of faith, with which you can extinguish all the flaming darts of the evil one;

What are those "flaming darts"? They are ideas, suggestions, thoughts, etc. Spiritual warfare is the battle for your mind.

ESV 2 Corinthians 10:4-5 For the weapons of our warfare are not of the flesh but have divine power to destroy strongholds. We destroy arguments and every lofty opinion raised against the knowledge of God, and take every thought captive to obey Christ,

For the unbeliever, all bets are off. Of course, the devil has little interest in the unbeliever as he is already spiritually dead or blind, but can and does possess people on occasion. Why?--I don't know... other than to say it is for the opposite reason God allows them temporary (and limited) activity. God allows it to His ultimate Glory (yes, even in the life of Job). Light is most beautiful when contrasted with darkness; Grace is sweetest when contrasted with Law's required judgment...

One last statement on devils and demons... As C.S. Lewis said in the preface to The Screwtape Letters,

There are two equal and opposite errors into which our race can fall about the devils. One is to disbelieve in their existence. The other is to believe, and to feel an excessive and unhealthy interest in them. They themselves are equally pleased by both errors ...