Pope Francis has slammed fundamentalism across religions, but warned that there are many Roman Catholics who believe they have "absolute truth" and are fundamentalists themselves.

The Vatican leader took aim at fundamentalism within the Church during an in-flight press conference at the conclusion of his three-country tour of Africa, calling it a "sickness that is in all religions."

"We Catholics have some — and not some, many — who believe they possess the absolute truth and go ahead dirtying the other with calumny, with disinformation, and doing evil. They do evil. I say this because it is my Church," the pope said, according to Catholic News Service.

He further added that fundamentalism is more idolatry than actual religion, warning that 'ideas and false certainties' can take the place of faith, love, and God.

"You cannot cancel a whole religion because there is a group or many groups of fundamentalists at certain moments of history," Francis added.

The Vatican leader shared that he is very pleased with his trip to Africa, and revealed that he prayed at a mosque in in Bangui, Central African Republic, riding around a Muslim neighborhood with the imam with him in the popemobile.

Francis said he was most impressed by "the crowds, the joy, the ability to celebrate even with an empty stomach" in his six-day trip to Kenya, Uganda and the CAR.

The pontiff focused on mending relationships and helping foster unity among Christians and Muslims during his trip, BBC News noted, and argued that Christians and Muslims are "brothers and sisters."

He also called for an end to fighting between Christian and Muslim communities in CAR, which over the past few years has seen a series of violent and deadly attacks.

"We are living at a time when religious believers, and persons of goodwill everywhere are called to foster mutual understanding and respect, and to support each other as members of our one human family. For all of us are God's children," Francis said in his pre-trip message.

He added that his visit to the African countries was "meant to confirm the Catholic community in its worship of God and its witness to the Gospel, which teaches the dignity of every man and woman, and commands us to open our hearts to others, especially the poor and those in need."

During his flight back to Rome, Francis again touched upon the ongoing U.N. climate conference in Paris, warning of great ills to the world unless leaders agree on steps to reduce harmful effects on the environment.

"We are on the verge of suicide, to put it strongly," Francis said, adding that every year the climate change situation "gets worse."

He admitted that he is not sure of the outcome of the conference, "but I can say this, it's now or never."