An Antipope is a false claimant to the Papacy, normally in opposition to the true Pope, the Successor of Saint Peter. The most common way an antipope comes into being is when a faction elects a man to the Papacy, while a true Pope is reigning. Since this is impossible, the election is most certainly invalid. The principle in the papacy is that the first claimant in time has the first right. In fact, until the 20th century, this is the only way an antipope was created, that is by a false Papal Election.The second way an Antipope can be created is by the attempted election of a heretic to the Papacy. Pope Paul IV fearing that this might be attempted one day, penned his famous infallible Bull, 'Cum Ex Apostolatus Officio', which states in part: "Further, if ever at any time it becomes apparent that any Bishop, even one conducting himself as a… Roman Pontiff before his promotion or elevation as a … Roman Pontiff, has strayed from the Catholic Faith or fallen into some heresy, or has incurred, encouraged or incited schism, then his promotion or elevation shall be null, invalid and void." The principle is that a heretic departs immediately from the Catholic Church, and as Saint Robert Bellarmine observes, "How can one be head of a Church of which he is not a member?" Thirdly, history has always considered the successor of an Antipope to be an Antipope. This can be seen in numerous examples, such as the Western Schism and the line from Antipope Anacletus II.The fourth way is by an alleged apparition or by any other manner than election with the sole possible exception of appointment by a predecessor, as has been alleged have occurred, when Saint Peter possibly designated Pope Saint Linus. This is a far more recent occurrence with the various Gregory's XVII, etc. 
