The Pope surprised observers in the US when he ditched a prepared speech about the challenges of family life to improvise a lively speech full of jokes and laughter as well as a serious message about God and love. But conservative evangelical and Catholic commentators have begun raising concerns about what they describe as leanings towards "moral relativism" on issues such as marriage and abortion.

Pope Francis was speaking in Philadelphia at a Prayer Vigil for the 8th World Meeting of Families taking place at the city's Benjamin Franklin Parkway when he went off script. The original, prepared text can still be read on the Vatican website.

After listening to six couples giving testimony about the joys and difficulties of family life, he set aside these remarks and spoke off the cuff, creating a headache for the thousands of journalists covering the event who, as is the norm at such occasions, had received the original text in advance and pre-written stories for publication after delivery. 

Speaking in Spanish, as he has for most of the visit to the US and Cuba, he opened with an amusing aside: "Once a child asked me – you know that kids ask difficult questions – he asked me Father, what did God do before creating the world?

"I assure you, I found real difficulty in answering the question. So I said what I'm now going to say to you: Before creating the world, God loved. Because God is love."

Pope Francis repeated his oft-stated concern about humankind "destroying" the beautiful world God created. And he said that of all the things in the world, the most beautiful was the family.

"Of course, it's not quite earthly paradise. There are still problems. Men and women, through the astuteness of the devil, have learned unfortunately how to divide themselves. And all that love that God gave, almost was lost."

He continued with the jokes, laughing as he riffed from possibly the oldest genre of all: "Father, you speak like that because you are single. Families have the difficulties. Families, we quarrel, and sometimes plates can fly. And children bring headaches. I won't speak about mother-in-laws..."

At the end, opera star Andrea Bocelli sang the Lord's Prayer and Pope Francis promised to see everyone at Mass today, after checking with the local Archbishop what time he had to be at church.

The speech was a triumph but behind the scenes, conservative commentators have been criticising Pope Francis.

Baptist Press reports that Southern Baptists are unhappy with his "lack of clarity on moral issues" and "church-state impropriety". 

Russell Moore, president of the Ethics and Religious Liberty Commission, said he was grateful the Pope spoke to Congress in New York about the dignity of all human life, whether the unborn, the elderly or the immigrant, as well as "the importance of the family in a free and flourishing society".

But he wished Pope Francis had been more direct about abortion culture and religious liberty.

 Albert Mohler, president of Southern Baptist Theological Seminary, said the pope's reference to abortion and marriage "was a very fuzzy and evasive approach that left many people wondering if he was actually talking about either abortion or marriage at all."

Bart Barber, pastor of the First Baptist Church in Farmersville, Texas, told Baptist Press: "For Congress to treat a church as though it were a state and the head of a church as though he were the head of a state runs contrary to basic First Amendment principles of disestablishment."

Conservative Catholics shared some of the evangelical concerns. Andrew P. Napolitano, a former judge of the Superior Court of New Jersey and senior judicial analyst at Fox News Channel, who had a rare invitation to watch the Pope address Congress, wrote: "The Church has taught for 400 years that abortion is murder. Because the victim of an abortion is always innocent, helpless and uniquely under the control of the mother, abortion removes the participants from access to the sacraments. Until now. Last week, Pope Francis, without consulting his fellow bishops, ordered that any priest may return those who have killed a baby in a womb to the communion of the faithful. He said he did this because he was moved by the anguished cries of mothers contemplating the murder of their babies."

He also said: "In the past month, without consulting his fellow bishops, the Pope has weakened the sacrament of matrimony by making annulments easier to obtain. The Church cannot grant divorces because Our Lord used his own words to declare valid marriages indissoluble. But it does grant annulments."