Original sentence: An antipope () or anti-pope is a person who has claimed to be Pope, the Bishop of Rome, but is not recognized as such by the Roman Catholic Church.Historically there have been various instances where more than one person has claimed the title of Pope at the same time, each opposing the other.Those whom the Church today rejects as illegitimate are considered antipopes
Edited sentence: An antipope () or anti-pope is a person who opposes a legitimately elected or sitting Pope and makes a significantly accepted competing claim to be the Pope, the Bishop of Rome and leader of the Catholic Church