The Vatican on Friday issued an unprecedented religious text co-written by Pope Francis and his predecessor Benedict XVI in which the two popes said faith should serve the "common good" but restated their opposition to gay marriage.
Francis paid tribute to pope emeritus Benedict XVI in the encyclical, saying that the ex-pontiff had "almost completed" the text before stepping down in a historic resignation this year and that he himself had merely added "further contributions."
The 82-page text stresses that there is no contradiction between the Catholic faith and the modern world and calls for more dialogue with scientists, other religions and non-believers.
It also restates the Catholic Church's position on marriage saying it should be a "stable union of man and woman."
"This union is born of their love, as a sign and presence of God's own love, and of the acknowledgement and acceptance of the goodness of sexual differentiation," reads the text.
While some passages in the encyclical have a more academic and ponderous feel characteristic of Benedict XVI, others contain the simpler expressions and brighter outlook of his successor.
Examples of Francis's input could be references to the need to protect nature and to sustainable development, as well as his oft-repeated phrase: "Let us refuse to be robbed of hope".
Gerhard Ludwig Mueller, head of the Vatican's Congregation for the Doctrine of the Faith, noted there were "differences of style, sensibility and accent" between the two popes in the text but said there was "substantial continuity of the message".
Francis and Benedict, who both live within the walls of the Vatican City and wear the white papal vestments, met publicly on Friday at a ceremony in the Vatican for the unveiling of a new statue.
Benedict became the first pope to resign of his own free will in 700 years in February and Francis was elected to succeed him in March as the first non-European pope in nearly 1,300 years.
The central message of the encyclical, entitled "Lumen Fidei" ("Light of Faith") is that faith should be considered a "common good".
"Its light does not simply brighten the interior of the Church, nor does it serve solely to build an eternal city in the hereafter, it helps us build our societies," it says.
The text also calls for a "return to the true basis of brotherhood", saying that the ideal of equality without faith "cannot endure".
In another passage the encyclical says that believers should be humble and not "presumptuous".
"As a truth of love, it is not one that can be imposed by force... Faith is not intransigent, but grows in respectful coexistence with others."
Encyclicals are papal circular letters addressed to the clergy of the Roman Catholic Church that are intended to summarise a pontiff's thoughts on a particular aspect of Church life.
Some of them have gone down in history as significant landmarks in Church history.
Pope Leo XII in 1891 published "Rerum Novarum" in which he undertook to engage the Catholic Church in social issues, denouncing living conditions for the working class and encouraging workers to organise themselves into associations.
In 1914, Benedict XV denounced the horrors of World War I in "Ad beatissimi apostolorum principis" and Pius XI in "Mit brennender Sorge" in 1937 condemned Nazi racism.
In Paul VI's "Humanae Vitae" in 1968, Paul VI condemned all forms of contraception, while John Paul II in "Evangelium Vitae" in 1995 called for opposition to laws legalising abortion and euthanasia.