Some call it the scariest movie ever made, and it's definitely a contender. “The Exorcist” starts out relatively slowly but then turns the tension up degree by degree, until it reaches an almost intolerable level.

Just about any horror fan will have seen this one already, but it always pays to see the great ones again. What makes The Exorcist such a successful movie is a combination of factors. One is that the writers took the trouble to set the horrific possession of a little girl in the context of otherwise believable events and people. The movie has texture, and there's a sense of a real and “normal” world that is being invaded from outside.

Another factor is the special effects, which are just understated enough to be really effective. There's no point in the movie where you find yourself unable to sustain suspension of disbelief due to cheesy pseudo-gore. This is a big problem with a lot of horror movies, and even more so now in the era of CGI.

The most effective element in the movie, though, is in its use of its own mythology. In traditional Christian theology, the powers of Hell are in no way a match for the power of God, and in theory a priest should easily be able to exorcize even the most determined demon. The mythology of “The Exorcist” is almost Manichean by contrast- the power of evil is seemingly unstoppable, and only the willing sacrifice of a heroic priest can turn it aside. This sense of evil as an ancient, supernatural and implacable enemy of humankind is what makes “The Exorcist” so effective.