The reference to the Antichrist is not intended to refer to the biblical Antichrist but is rather an attack on the "slave morality" and apathy of Western Christianity. Nietzsche's basic claim is that Christianity (as he saw it in the West) is a poisoner of western culture and perversion of the words of and practice of Jesus. In this light, the provocative title is mainly expressing Nietzsche's animus toward Christianity, as such. In this book, Nietzsche is very critical of institutionalized religion and its priest class, from which he himself was descended. The majority of the book is a systematic, logical and detailed attack upon the interpretations of Christ's words by St. Paul and those who followed him.
It can therefore be argued that it does a disservice to the English-speaking reader to translate the title as The Antichrist at all. As has been said the German title Der Antichrist is open to two interpretations ('Antichrist'/'Anti-Christian') but the English The Antichrist is not, and thus the question becomes: does that title reflect Nietzsche's use of the German word in the text? In fact, Nietzsche employs the word Antichrist at only one point, and there its sense is clearly 'Anti-Christian.' He uses its plural Antichristen once also, and again the meaning is 'anti-Christians.' In fact, at no point in the text does Nietzsche use any form of the German word Christ other than to mean 'Christian.' On this approach, a reader would be better readied for book's content were its title rendered "The Anti-Christian", which more accurately identifies the author's chosen foe. After all, even to acknowledge the existence of an Antichrist, let alone to portray oneself as one, presupposes the existence of a Christ, of a Messiah. Nietzsche recognized no such entity. His argument was entirely with those who would attempt to make a Christ, a Messiah, out of Jesus of Nazareth, which is to say, with "Christians."
On the other hand, Walter Kaufmann considers The Antichrist the more appropriate way to render the German, in spite of its ambiguity: the "translation of the title as 'The Antichristian' ... overlooks that Nietzsche plainly means to be as provocative as possible."
Consider too that the title itself is part of the polemic Nietzsche makes here against Ernest Renan. Renan's 1873 L'antéchrist saw an "authorized German-language edition" published the same year under the title, Der Antichrist.