I believe in Satan and demons. Anyone who believes the Bible must also acknowledge the existence of evil and the servants of such. However, I don't think of the Devil as the stereotypical red-skinned, cloven-hooved, pitchfork-wielding character you see in cartoons. That image came out of the Medieval times and doesn't match with what the Bible says abut him.
Satan is beautiful. He is subtle. He is manipulative. He works on your fears and desires. He's not the Faustian character who gets you to sign away your soul on a piece of paper. Rather, he lies and tricks you into doing things that will result in your soul going to him anyway, and makes you think it's fun and your idea all along. Most people are prepared to resist the greater evils, but aren't as cautious about the smaller ones. Think of things like lying and lustful thoughts as the "gateway drug" of sin. Just like drugs such as marijuana can lead to harder drugs such as crack, "minor" sins can lead to comfort in this area and eventually to serious sins.
As part of my ministry with Fans For Christ I attend sci-fi/fantasy/comic conventions, talk about Jesus there and hold church services. There are plenty of things at these events that are fine for Christians to like and be around. I'm a big fan of these genres myself, and an active comic-book geek as well as a role-player. But there are also some very anti-Christian and even demonic forces at these events. It takes discernment to tell the difference, and it's not always a clear choice. Satan wants it to be this way, with you thinking that you're really not doing anything wrong, thus leading you down a dangerous path. If you reject God your soul is his...and you don't even have to sign on the dotted line in blood.
So how do we resist these influences? Pretty simple, actually. You read the Bible, pray, and surround yourself with Christians. We are all bombarded by influences every moment of the day. The media and advertising vie for our attention, trying to convince us of their point of view. We have work and family issues that try to sway our thoughts in one direction or another. If you have a certain type of thought or influence around you all of the time, it's human nature for your mind and attitude to gravitate in that direction. Surrounding yourself with things of God helps deflect the negative influences. And it's critically important.
I have a friend who is in the process of trying to divorce his wife in the year that they would celebrate their 20th anniversary. He isn't in church, though says he's a Christian. In the last year he has been in a job that keeps him away from his family. When he's home he's heavily involved in things that aren't anti-Christian, but still are very worldly and secular. He has no Christian influence in his life to guide his thoughts in a Christ-like direction. And I think this is affecting his marriage. I also fear for where he will end up if he continues down this path.
A story is told of a crotchety old man who stopped going to church. He developed a bad attitude towards people and his wife was worried about him. After he started getting meaner and more depressed, she contacted her pastor. She explained what was going on but said that he wouldn't listen to her about going back to church. The pastor came over one day and found the old man sitting near the fireplace. Without a word the pastor sat in a nearby chair and stared at the fire. The two men sat in silence for several minutes before the pastor got up and used some tongs to take an ember out of the fire. He placed the glowing coal on the brick in front of the fireplace and then sat down again. The men watched the glow fade, and then remain dark for several minutes. The pastor got up again, picked up the cold coal with his bare hand, and threw it back into the fire where it quickly started glowing warm again. The old man nodded and sighed. "See you at church this Sunday, pastor."
If we don't keep ourselves in the fire of Christ and surround ourselves with other Christians, the Bible, and prayer, our faith will cool. That is when Satan will take us and we won't even realize it.