The subject matter has surprised Vatican officials, expecting the Pontiff to address more problematic issue facing the church especially given Benedict XVI was the Vatican's chief doctrinal watchdog.
Encyclicals are used by popes to communicate their thoughts to the wider Catholic community and indicate what the pontiff’s major concerns are.
The Pope on sex
In his first authoritative text Benedict XVI lamented what he describes as the separation of sex and selfless, spiritual love.
"Eros, reduced to pure 'sex', has become a commodity, a mere 'thing' to be bought and sold, or rather, man himself has become a commodity," he said.
The Pope added erotic love needs to be "disciplined and purified” for it to provide us with a foretaste of “agape”, the Greek word for unconditional, spiritual love. He wrote “eros” and “agape” become most unified in marriage between man and woman.
Works of charity
In the 71-page letter Benedict XVI proclaimed that the Church and charity were intrinsically linked and that the Church must “not remain on the sidelines” in the fight for social justice.
"For the Church, charity is not a kind of welfare activity which could equally well be left to others, but it is a part of her nature, an indispensable expression of her very being," the Pope said.
He cautioned that the Church must never use charity to convert the vulnerable to Catholicism. "Love is free, it is not practised as a way of achieving other ends," he wrote.
“Those who practise charity in the church's name will never seek to impose the church's faith upon others. They realise that a pure and generous love is the best witness to the God in whom we believe and by whom we are driven to love," he said.
Benedict XVI warned that the Church cannot “and must not” take up the political battle to secure a just society. "A just society must be the achievement of politics, not of the Church," he said.
"She cannot and must not replace the State. Yet at the same time she cannot and must not remain on the sidelines in the fight for justice," he cautioned.
Reaction to encyclical
Prominent papal critic and German theologian Hans Kueng welcomed the encyclical but said the pope failed to mention that charity should be extended to those who had fallen from grace with the church’s teachings.
Mr Kueng pointed to loving couples who use contraception and those who divorce and remarry. "One would hope that beyond the Roman Catholic congregation of the faith there is a congregation of love," he said.
US Cardinal William Joseph Levada, who replaced Benedict as head of the Vatican Congregation for the Doctrine of the Faith, said the document "offers us a vision of love for the future and the ecclesiastical duty to work for charity."
The head of the Catholic charity Caritas, Denis Vienot, told AFP the encyclical was "an extraordinary appeal to lay Catholics to engage with society" including by becoming involved in politics.