On Christmas Eve, Pope Francis, “a pope for the poor”, the first Jesuit elected to the papacy, returned to what has become a recurring theme of his first two years in office – the venality of power and extreme wealth and the lessons wrenched from poverty. The light in the sky at the birth of Christ, he said in his address, was not seen by “the arrogant, the proud, by those who made laws according to their own personal measures”, but by “the unassuming”.

Some of the cardinals, bishops and priests who run the Holy See, whom earlier in the week had been the recipients of the pope’s blistering attack on the 15 ailments that plague the Vatican, must, again, have felt the sting of criticism. On Monday, the 78-year-old had railed against the upper echelons of the church for being infected by careerism, backstabbing and hypocrisy. The pope criticised “the terrorism of gossip” that could “kill the reputation of our colleagues and brothers in cold blood ”. Officials, he said, suffered from “spiritual Alzheimer’s”, they were rivals and boastful, they sought worldly profit and had become hardened to others. His reception was frosty.

Pope Francis, born in Argentina of Italian heritage, is a man engaged with the world. He recently achieved a major coup, brokering the restoration of relations between the US and Cuba. At Davos, he chided the rich for neglecting the “frail, weak and vulnerable”. A neoliberal he isn’t. In Evangelii Gaudiium (The Joy of the Gospel), his first major work after he became pope, the pontiff wrote: “The worship of the ancient golden calf… has returned in a new and ruthless guise in the idolatry of money and the dictatorship of an impersonal economy.”

On the Sicilian island of Lampedusa, 70 miles from the Tunisian coast, he demanded more action to save African refugees drowning in the seas in their quest for a better life. “In this globalised world,” he said, “we have fallen into globalised indifference.”

Next year, he will intervene again in the secular world. Today, the Observer reports his intention to write a letter to the world’s 1.2 billion Catholics, give an address to the UN General Assembly and call for a summit of the world’s main religions. He will also issue a rare papal encyclical in March on climate change and human ecology, all part of an attempt strongly to influence the UN climate change meeting in Paris in 2015 to achieve a universal commitment to reduce carbon emissions. “Eco-theology” alarms creationists and divides the Catholic church. Opponents, including some in the Vatican, condemn it as “un-biblical”. Pope Francis sees economic inequality and the plundering of the Earth’s resources as part of a theme. He has said: “In this system, which tends to devour everything which stands in the way of accrued profits, whatever is fragile like the environment is defenceless against the interest of a deified market, which becomes the only rule. ”

Pope Francis, the leader of one in six of the world’s population (41% in Latin America), is undoubtedly a man of change and with huge potential leverage. He has appointed eight cardinals from five continents to reform the Vatican bureaucracy. He has sacked cardinals running the Vatican bank. He has removed key conservatives from the body that appoints bishops. He has said he wants a collegiate, consensual style and stripped his own office of pomp and some of its pomposity. He replaced the papal Mercedes with a Ford Focus. On Holy Thursday, soon after his election, instead of washing the feet of 12 priests, he visited the Casal del Marmo jail, a Rome juvenile detention centre. He washed and kissed the feet of 12 inmates, including two Muslims and two women. Simplicity, austerity, humility – and inclusion – are much of his message.

He is a reformer but is he a moderniser and a radical? He is viewed as conservative on sexual matters – against contraception, euthanasia and abortion. Argentinian president, Cristina Fernández de Kirchner, calls his position on same sex marriage “medieval”. But that may be too simplistic. Asked about his position on homosexuality, he replied: “If a person is gay and seeks God and has good will, who am I to judge?” Pope Francis’s model is the reforming Pope John XXIII. The latter’s Vatican II in the 1960s sparked a crisis in Latin America as the Catholic hierarchy allied with oligarchs and dictators were challenged by priests and nuns at the grassroots fighting for social justice. Pope Francis said to Antonio Spadaro, editor of La Civiltà Cattolica: “Many think that changes and reforms can take place in a short time. I believe that we always need to lay the foundations for real, effective change. And this is the time of discernment. Sometimes discernment instead urges us to do precisely what you had at first thought you would do later.”

Jorge Mario Bergoglio was born in 1936 in Buenos Aires, the son of an accountant. He began his working life as a lab technician, joined the Society of Jesus as a novice in 1958 and was ordained to the priesthood in 1969. He became archbishop of Buenos Aires in 1998. In The Great Reformer, Austen Ivereigh describes how the pope navigated the years of the Argentinean military dictatorship: the “dirty war”, in which thousands were killed, tortured and “disappeared”. He was accused of handing two Jesuit priests over to the junta by declining to endorse their work in the slums. Ivereigh says the charge is false. The priests were arrested and tortured but as a result of Bergoglio’s intervention, they were released. Bergoglio has said he learned lessons. “My authoritarian and quick manner… led me to.... be accused of being ultraconservative.”

The Vatican still has its scandals, most notably the incalculable hurt caused to tens of thousands by helping to cover up decades of child sexual abuse, but Pope Francis appears to be trying to bring transparency and accountability. He speaks on behalf of the poor but how much is he prepared to challenge the power of those, including the church, with an excess of influence and wealth?

Soon after his election, Pope Francis invited Gustavo Gutiérrez, founder of liberation theology, to Rome. As leader of the Jesuits in Argentina in the 1970s and 1980s, the pope was part of the suppression of the movement that aimed to place the church on the side of the poor in the struggle for social change. Rome saw the movement as a cover for Marxist class struggle. Now, as pope, Francis says liberation theology can no longer “remain in the shadows”. Another key figure, Leonardo Boff, is contributing to the pope’s thoughts on eco-theology. It’s impressive that a man approaching his 80th year is still learning, still prepared to push for change. How far, as yet, is unclear.

Following the washing of the feet of young offenders, Gutiérrez was asked if Pope Francis was a man of style or substance.

“If it is just for show,” Gutiérrez replied. “I say keep on showing it.”