There are people who don't believe in the devil. In fact there is a great line from the book The Screwtape Letters by C.S. Lewis in which Satan says that his greatest victory was convincing people that he didn't exist.
I believe that satan is in the world and working for the ruin of souls. I believe that no one, not even the strongest Christian is completely safe from satan's grasp. I also believe that satan can only have the room in our lives that we allow him to have. The problem is that all it takes is for us to let our guard down a little and satan will slip into our world.
Satan is the author of all lies. His lies can seduce us because they make us forget the virtues that God has given us to live under. Virtues like patience and temperance and humility go right out the window when satan appeals to our pride and covetousness.
We are taught as a society that we deserve certain things. We want what we want and we aren't willing to sacrifice or wait or even consider that we don't deserve or need whatever it is. We always want what is best for us even if it inconveniences or harms others.
It is easy to be seduced by the evil one. We have to guard ourselves especially from becoming inordinately angry over some slight or harm we feel we have suffered. Anger is a vice that can really make us incapable of doing God's will in our lives. Anger can hamper our ability to think rationally or act with humility and mercy.
I guess what I wanted to say was that satan is out there, and we have to be careful every moment that we aren't seduced by his lies into doing something that would be harmful to ourselves or to someone else.