One thing that's distressing me is, well kind of, a big problem ever since childhood. I have been scared of demonic stuff ever since I can remember and movies like "The Omen" just freak me out. There was a period of time when I was convinced that the devil was trying to possess my body while I was sleeping, which gave me a lot of stress, but that didn't last long. This week I read several articles on how Satan wants us to lose our salvation and is trying his hardest to keep us down and I'm really scared. What can I do to keep Satan from causing me to fall away from God, and making me distress (combined with the fear of evil spirits possessing people).
Typically people fear things produced by their imaginations. We get worked up over what might happen; then, when we are forced to face reality, we find that it wasn't nearly as bad as we feared it might have been. For more on this, see the article "Overcoming Fear."
Your fear of demon possession is an example of a fear produced by imagination. I can say this with confidence because God long ago stated that demons would not stay around. ""In that day a fountain shall be opened for the house of David and for the inhabitants of Jerusalem, for sin and for uncleanness. It shall be in that day," says the LORD of hosts, "that I will cut off the names of the idols from the land, and they shall no longer be remembered. I will also cause the prophets and the unclean spirit to depart from the land"" (Zechariah 13:1-2). The time when cleansing from sin and uncleanness came when Jesus died upon the cross. The apostles and prophets worked to complete the recording of the Word of God, and when the perfect will of God was delivered, prophecy came to an end (along with other miraculous gifts). "Love never fails. But whether there are prophecies, they will fail; whether there are tongues, they will cease; whether there is knowledge, it will vanish away. For we know in part and we prophesy in part. But when that which is perfect has come, then that which is in part will be done away. When I was a child, I spoke as a child, I understood as a child, I thought as a child; but when I became a man, I put away childish things" (I Corinthians 13:8-11). With the end of prophecy also came the end of demons' direct participation in the world.
This is not to say that Satan and his minions are not still trying to influence the world toward evil. But it means that no demon can possess a person against his will. A person might willingly follow after the influence of a demon, but that is about as far as it goes. "Now the Spirit expressly says that in latter times some will depart from the faith, giving heed to deceiving spirits and doctrines of demons" (I Timothy 4:1). But Satan can't make you do evil. He can only make evil look tempting and try to persuade you to do evil voluntarily (something Satan is quite successful at getting people to do). "But each one is tempted when he is drawn away by his own desires and enticed. Then, when desire has conceived, it gives birth to sin; and sin, when it is full-grown, brings forth death. Do not be deceived, my beloved brethren" (James 1:14-16).
God has put limits on what Satan can do. He can't put you in a tempting situation that is so strong that you can't help but sin. "No temptation has overtaken you except such as is common to man; but God is faithful, who will not allow you to be tempted beyond what you are able, but with the temptation will also make the way of escape, that you may be able to bear it" (I Corinthians 10:13). Therefore, Satan can't come in and take over your body. God doesn't allow him to have this power.
Yes, Satan is trying to bring Christians down. That is why we must always be on our guard. Satan is quite clever about hiding sin in very appealing packages. "Be sober, be vigilant; because your adversary the devil walks about like a roaring lion, seeking whom he may devour. Resist him, steadfast in the faith, knowing that the same sufferings are experienced by your brotherhood in the world" (I Peter 5:8-9). How do we resist Satan? The same way Jesus did in his temptations -- through the word of God (Matthew 4). Each time Satan tried to tempt Jesus to sin, Jesus responded with what God taught on the matter. In other words, Jesus remained steadfast in the faith. "Adulterers and adulteresses! Do you not know that friendship with the world is enmity with God? Whoever therefore wants to be a friend of the world makes himself an enemy of God. Or do you think that the Scripture says in vain, "The Spirit who dwells in us yearns jealously"? But He gives more grace. Therefore He says: "God resists the proud, But gives grace to the humble." Therefore submit to God. Resist the devil and he will flee from you. Draw near to God and He will draw near to you. Cleanse your hands, you sinners; and purify your hearts, you double-minded" (James 4:4-8).
"I have written to you, young men, because you are strong, and the word of God abides in you, and you have overcome the wicked one. Do not love the world or the things in the world. If anyone loves the world, the love of the Father is not in him. For all that is in the world--the lust of the flesh, the lust of the eyes, and the pride of life--is not of the Father but is of the world. And the world is passing away, and the lust of it; but he who does the will of God abides forever" (I John 2:14-17).