Speaking about the Roman Catholic Church he warned it cannot become like a country under a dictatorship.
The Pope, 78, has previously said that he would be ready to resign instead of ruling for life if he felt he could not continue running Church for health or other reasons.
The Catholic Church includes around 1.2 billion-members worldwide.
Speaking in an address to some 30,000 people at an inter-denominational rally of Christians in St. Peter's Square, he said: "Let's be clear. The only one who cannot be substituted in the Church is the Holy Spirit.
"There should be a time limit to positions (in the Church), which in reality are positions of service."
The Argentinian pontiff stressed his comments did not apply solely to the clergy and said: "It is convenient that all (positions) in the Church should have a time limit.
"There are no leaders for life in the Church.
"This occurs in some countries where a dictatorship exists."
In February 2013, the Pope's predecessor, Pope Benedict XVI became the first pontiff to resign in 600 years.
Pope Francis said what Benedict, now known as Pope Emeritus, did "should not be considered an exception, but an institution," speaking in a TV interview.
During the same interview he said there should be no official age to resign though.