THE traditional Christian teaching that marriage is a lifelong and exclusive union between a man and a woman has been reasserted by Pope Francis in his first encyclical.

He published Lumen Fidei: The light of faith after some 100 days in office, focusing on the subject of faith. But, at a time when countries around the world - including the UK - are extending marriage to include same-sex couples, the Pope makes explicit the belief that marriage is reserved only for heterosexual couples.

"The first setting in which faith enlightens the human city is the family," Pope Francis writes in the encyclical, which was signed on 29 June, the feast of St Peter and St Paul, and published on 5 July.

"I think first and foremost of the stable union of man and woman in marriage. This union is born of their love, as a sign and presence of God's own love, and of the acknowledgment and acceptance of the goodness of sexual differentiation, whereby spouses can become one flesh, and are enabled to give birth to a new life, a manifestation of the Creator's goodness, wisdom, and loving plan.

"Grounded in this love, a man and a woman can promise each other mutual love in a gesture which engages their entire lives and mirrors many features of faith.

"Promising love for ever is possible when we perceive a plan bigger than our own ideas and undertakings; a plan which sustains us and enables us to surrender our future entirely to the one we love."

Lumen Fidei completes a trilogy on the theological virtues started by Pope Benedict XVI, who issued encyclicals on charity, in the publication of Deus Caritas Est (God is Love), in 2005; and hope, with Spes Salvi (Saved by Hope), in 2007.

At a press conference at the Vatican, the president of the Pontifical Council for Promoting the New Evangelization, Archbishop Rino Fisichella, explained that Pope Benedict had intended to write an encyclical on faith, and publish it at the end of the Year of Faith, which he opened last October - on the 50th anniversary of the opening of the Second Vatican Council - and which will run to the end of November.

"However, history took a different turn, and this encyclical is now offered to us today by Pope Francis," he said. "It must be said, without hesitation - while Lumen Fidei resumes some of the intuition and themes typical of the ministry of Benedict XVI, it is fully Pope Francis's text.

"Here we encounter his style . . . the immediacy of his expressions, the rich images he uses, and the peculiarity of his use of quotations from ancient and modern authors, make this text a true introduction to his teaching," Archbishop Fisichella said.

In the first of four sections, the encyclical unpacks the meaning of faith from a biblical perspective, beginning with Abraham, before moving on to Jesus. The second chapter deals with philosophical issues, including the relationships between faith and truth, and between faith and reason.

The third deals almost exclusively with the importance of evangelisation as a response to the gift of faith by the Church; and the fourth deals with the relationship between Christians and the world, and how that is shaped by faith.

At the press launch, the prefect of the Vatican Congregation for Bishops, Cardinal Marc Ouellet, said that faith played a vital part in establishing freedom and justice in civil societies, and in conflict-resolution in nations afflicted by violence and war. "The tendency to confine faith to the private sphere is calmly but decisively rejected here," he said.

Lumen Fidei was welcomed in England with a pastoral letter issued this week by the Roman Catholic Bishop of Portsmouth, the Rt Revd Philip Egan. "Its basic message is that faith is a gift from God, which enables us to see the world and our place within it as it really is," Bishop Egan said. "Faith is seeing with the eyes of Christ. It is knowledge born of love, the love of God poured into our hearts. Christian faith and natural reason go together, complementing one another, and one without the other leads to distortion."

Bishop Egan said that he was particularly concerned about the social consequences of the spread in British society of "scientism" - the belief that truth can be obtained only through empirical data.

He said he hoped that the study of Lumen Fidei would "once and for all knock on the head false perceptions about faith and reason, and enable us, as people of Christian faith, not only to take fresh heart, but also to offer others in our society a much-needed corrective.

"We believe that faith and reason go together. They are two forms of knowing, and essentially there is no conflict between them."

The text of the encyclical is available in full at www.vatican.va/holy_father/francesco/ (encyclicals).