The Antichrist should be understood as a work of theology, since by nature it only treats religious questions. Within that category, the work functions like horror. The book is designed as its title suggests, to the chagrin of Christians. Not only is Nietzsche out to make some pretty offensive claims (especially in Europe at the time), he backs them up with air-tight arguments pointing to many problems with religion that are more or less correct. The book is perplexing in the way it undoes a person's sense of religious faith.
Nietzsche's work is the ultimate test of faith, because it reminds the reader of all the problems religion implies, problems they ignored in the formation of their religious beliefs. Nietzsche seems to ask the reader, "When you became a Christian, do you remember sacrificing some of your intellect?" His assumption is that anyone who bases even a portion of their life on blind religious belief has made a serious compromise in their intellect.
That's not the same as atheism, either. It's straight nihilism. Nietzsche could very easily have written the same kind of book to disturb an atheist (and he often does it in other works). It's the act of religious faith itself that Nietzsche disdains. He regards the belief in the Bible as a form of intellectual sickness, like a disease that spreads person to person as a parasite, preying on human suffering and the human willingness to "believe" something for purposes of wish fulfillment.
