During his four years in office, Pope Francis has declared evolution is real, assured atheists they don't have to believe in God to go to heaven, and spoken out against the rise of populism in the west.
Encouraging Catholics to practice what their religion teaches, in his often impromptu sermons, the 80-year-old pontiff has also likened the sexual abuse of children by priests with "satanic mass" and told his cardinals not to act as if they were "princes".
Here is a collection of his best quotes:
Evolution and Big Bang
God is not "a magician with a magic wand" and the theories of evolution and the Big Bang are real, the Pope declared in 2014.
Explaining that they were not incompatible with the existence of a creator, he argued instead, that they “require it”.
“When we read about Creation in Genesis, we run the risk of imagining God was a magician, with a magic wand able to do everything. But that is not so,” the pontiff said.
“The Big Bang, which today we hold to be the origin of the world, does not contradict the intervention of the divine creator but, rather, requires it.
“Evolution in nature is not inconsistent with the notion of creation, because evolution requires the creation of beings that evolve.”
Experts interpreted his comments as an attack on the "pseudo theories" of creationism and intelligent design which some argued his predecessor, Benedict XVI, had espoused.
But Pope Francis' comments followed those of Pope Pius XII who actively welcomed the Big Bang theory and Pope John Paul II who suggested evolution is "more than a hypothesis" and is an "effectively proven fact".
Pope Francis has called on the world to work to resolve the refugee crisis, saying the world needs bridges instead of walls.
“Barriers create divisions instead of promoting the true progress of peoples, and divisions sooner or later lead to confrontations,” he said.
The pontiff backed up his words with actions, when he took three families of Syrian refugees back to Rome after offering them a place on his plane.
Atheists go to heaven
Shortly after taking office in March 2013, the Pope said non-believers would be forgiven by God if they followed their consciences.
"You ask me if the God of the Christians forgives those who don’t believe and who don’t seek the faith," he said. "I start by saying – and this is the fundamental thing – that God’s mercy has no limits if you go to him with a sincere and contrite heart.
"The issue for those who do not believe in God is to obey their conscience. Sin, even for those who have no faith, exists when people disobey their conscience.”
'Better to be an atheist than hypocritical Catholic'
Pope Francis has also claimed that it is better to be an atheist than one of "many" Catholics who lead a hypocritical double life.
Criticising some members of the Church, he said: "It is a scandal to say one thing and do another. That is a double life. "There are those who say 'I am very Catholic, I always go to Mass, I belong to this and that association'."
Some of these people should also say: "'My life is not Christian, I don't pay my employees proper salaries, I exploit people, I do dirty business, I launder money, [I lead] a double life'," he added.
"There are many Catholics who are like this and they cause scandal," he said. "How many times have we all heard people say 'if that person is a Catholic, it is better to be an atheist'."
Less than two months after his election, he said Christians should see atheists as good people if they do good.
'Populism is evil'
Pope Francis has warned about the dangers of rising populism in western democracies.
“Populism is evil and ends badly as the past century showed," he said.
When he was asked if he worries about the rise of populism in the United States and Europe, he said people should not repeat the same mistakes as in the 1930s, when they turned to “saviours” to resolve economic and political crises only to end up at war.
“Crises provoke fear, alarm. In my opinion, the most obvious example of European populism is Germany in 1933,” he said. “A people that was immersed in a crisis, that looked for its identity until this charismatic leader came and promised to give their identity back, and he gave them a distorted identity, and we all know what happened.
“In times of crisis, we lack judgement, and that is a constant reference for me... That is why I always try to say: talk among yourselves, talk to one another."
Fake news is like getting aroused by faeces
The pontiff said writing fake news and stories about scandals is like being sexually aroused by excrement.
He said spreading fake news is "probably the greatest damage that the media can do", calling it "a sin" to do so instead of educating the public.
"I think the media have to be very clear, very transparent, and not fall into — no offence intended - the sickness of coprophilia, that is, always wanting to cover scandals, covering nasty things, even if they are true," he said.
"And since people have a tendency towards the sickness of coprophagia, a lot of damage can be done."
He later apologised for his use of a precise psychological term, which describes when people are aroused by excrement.
Severe punishment of paedophiles
Despite his progressive reputation, Pope Francis has been criticised for his handling of child rape scandals within the Catholic Church.
He has called for the "severe punishment" of paedophiles.
But it was revealed earlier this year that he had quietly recommended that the church sentence paedophile priests to a lifetime of prayer instead of serving time in jail.
In one case, Pope Francis reduced the penalty for Reverend Mauro Inzoli to a lifetime of prayer after he was convicted for sex crimes against five children as young as 12.
It was said to be one of several times that he had overruled recommendations from his own advisory council and reduced a priest's sentence.
The priest in questions had been reassigned to a school in Argentina after being accused of sexually abusing children in Italy.
A senior Australian Catholic official has also warned the Pope may be "backsliding" in his crackdown on paedophile priests.
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