And, he hates above all God and the plan of salvation which he himself has rejected. He will do anything in his power to derail God's plan and to snatch people away from God's love, to make other suffer as much as he suffers in Hell. Therefore, whenever the Prince of Lies is present with even the smallest opening, he takes advantage of it and rushes into the person. So, a propensity to do evil leads to a fascination with evil. This in turn can develop into obsession with evil.
Obsession becomes, if unchecked, possession .
Once he is in, he can only be driven out by prayer and fasting, as Christ says. I will tell you that every Diocese is required to appoint three priests as Exorcists, that this Diocese does have three such priests, and that they are never allowed to perform an exorcism without an extensive process to determine if there really is a possession. Further, they need the permission of the bishop and they may not work alone.