First and foremost, Evil exists in the world today just as it always has in a very real and tangible form. The only real difference is that mankind as a whole refuses to believe in the existence of true evil and this attitude allows it to flourish virtually unchecked. It is also important to note that many exorcists approach your situation with a great deal of skepticism. This is not only normal and healthy but mandatory. Rituals of exorcism performed on people with serious mental or psychological disorder can cause many problems. Therefore, most exorcists will set out trying to rule out demonic oppression or posession. Do not be discouraged by this, trust in the clergy you are dealing with, and feel free to ask them questions. Exorcists are trained to know when it is demonic activity. You must also understand that most exorcisms are not a quick-fix. Some cases can be prolonged requiring numerous sessions and a constant prayer regimen. If however, the clergy assisting you is charging you money for this process, you should investigate an alternate exorcist.
For those that have not encountered this type of evil, I am sometimes envious. This evil of which I speak is terrifying beyond the ability to verbalize. It is an energy that effects a human physically, spiritually and mentally. Prolonged exposure to this evil is both toxic and intoxicating for some. It is impossible to break free of this phenomena without the aid of the one True God's grace.
Why does God allow evil? Simple. God prefers man to worship him of his own free will, relying on faith rather than proof. God allows evil to exist so that man can exercise the freewill god has given him. Without evil, man would be "robotic" followers never having a real choice in the matter. This same danger comes into play when God provides proof of his existence. Once a person has tangible evidence of the existence of God, it is no longer a matter of faith to follow him. Without temptation, there can be no forsaking the temptation to follow the ways of righteousness.
Why does God allow evil to oppress, possess or influence man? This is also simple. God doesn't allow this, man does. Most of the time demonic oppression and influence have been invited, however inadvertently, by man himself. Possession is a deeper symptom of oppression which is also sometimes invited upon man himself.
Lets now talk about the different types of demonic influences.
Influence - itself is just that. A Presence of evil that tempts man to deepen his walk with evil and separate himself from God's grace. Influence can be a desire to do wrong, a thought to deviate from good, and belief that succumbing to the sinful nature of man is "ok" or "natural". Influence lays the groundwork for other experiences. Most influence can be stopped cold with prayer, fasting and a return to a pious Christian life. You may notice the presence of foul odors (without being able to find the source), you may hear sounds that you cannot locate the source for. Knocking sounds are common.
Oppression - this is malicious and aggressive assault on man by evil entities. This can manifest itself as virtually anything from physical health problems, to loss of income, mental capacity or a distinctive shift in behavior and attitude. Oppression almost always follows closely on the heels of influence although it can sometimes occur without noticeable influence first. It is the direct result of surrender of free will to evil and an almost sure sign that a Christian has strayed outside of grace. Fortunately, through the blood of Christ, we are able to repent and mend our ways allowing god to protect us from oppression. Prayer, support from your church, return to a pious Christian life, and sometimes help from a priest is needed and will remove oppression from your life.
Possession - This is a complete take over of the body and life of a human by a demon or demons. This frequently follows oppression although it can occur without noticeable signs of oppression. This is a RARE occurrence although it is more common in this day and age than many prefer to admit. There are several mental illnesses and psychological conditions that can appear as possession, for this reason it is vital that a person believed to be "possessed" should first receive psychological evaluation from a professional prior to an exorcism and in fact, the Servants of Solemnity will require proof of a psychological evaluation prior to performing any exorcism. The Servants of Solemnity prefers to have your therapist present during the exorcism but we recognize this is not always possible or practical. Psychologists will charge an hourly rate, whereas a true exorcist will not. Servants of Solemnity are prohibited from performing an exorcism without approval from the Marshall of their region and where possible at least one bishop of the church they are a member of. Possession will require an exorcism ranging from a mild or ordinary exorcism to a radical and very taxing ritual. It is always best to have prevented this occurrence at one of the first two steps. Before an exorcism can work, the possessed must want to renounce evil and return to a pious Christian life.
To eliminate these phenomena from your life, it is best to begin with an analysis of your life, lifestyle and spiritual walk. If you find deviance from a Pious Christian life everything will seem very clear to you. Repentance and a renunciation are the next essential steps.
Sometimes, healing must be conducted in your home in an effort to eliminate the evil presence that has been allowed to take up residence. This can be as simple as a house blessing or as complex as holding a mass in your home. It is not uncommon for the process to take a while. It is unlikely that the events began overnight, it stands to reason that they are just as unlikely to cease overnight. It is important to contact an exorcist, priest or a member of the servants of solemnity to make arrangements for this. You cannot rely on a psychic medium, or a new age ghost hunter to do this for you. While many psychics and mediums genuinely mean well, the methods they use to attempt to clean a home are ungodly and often tools of the very creatures you are trying to rid yourself of. These people are ill-equipped to perform the task properly and will most likely cause more harm than good. A restoration to a state of grace, regular use of sacramentals, and the sacraments will rid you of these experiences although not usually overnight.