There are three sources of demonic intrusion into our lives. They are 1) our own sinful nature which conforms to the ways of Satan and the demonic; 2) the world, or the cultures in which we live, which are set up, controlled, and inspired by Satan and the demonic; 3) direct transferrence of thoughts into our minds by demonic spirits.
Demonic spirits know our individual moral weaknesses, and also where our faith is weak. By simply "dropping" thoughts or suggestions into our minds that are consistant with the personal weaknesses of our sinful nature and desires, they can lead or inspire us to sin or to keep returning to sinful habits and addictive behavior.
In short, demonic spirits work by bringing up the nasty stuff that's still deep down within us until our death and resurrection.
Whatever the source of inspiration to sin, it is essential to resist every temptation. In doing so the "devil," if present, will flee. By resisting temptations, one by one, we act out our love and devotion to Christ, proving that our desire for him is greater than our desire for the empty pleasures of sin.
Resisting temptation is part of what it means to deny ourselves for the sake of following Jesus.
