The term ONE OF 13 POPES can be summarized by two crossword solutions: LEO (3 letters) and LEON (4 letters). LEO is the name of a pope who reigned from 440-461 AD. He was the first pope to have been given the title 'the Great' and is known for his significant contributions to the development of the Catholic Church. Leon was the name of a pope who reigned from 590-604 AD. He is known for his efforts to combat heresy and for his support of the Gregorian chant.