Pope Francis has called for a creative, humble approach as the Vatican ponders why many in the Catholic Church have gone their own way on sexual and family issues like contraception, pre-marital sex and divorce.

A Mass in St. Peter's Basilica on Sunday opened a two-week meeting of 200 cardinals and bishops from around the world, who know that many in their flock, despite considering themselves Catholic, defy church teaching on sexual matters. The most contentious issues include bans on artificial birth control and on Communion for divorced faithful who remarry.

The Catholic church considers divorced Catholics who remarry to be living in sin. It says those Catholics can only receive Communion by abstaining from sex with their new spouses.

Feedback from a survey Francis had his bishops give last year confirmed the gap between pastors and parishioners on sexual matters.

Without elaborating, Francis lamented that "pastors lay intolerable burdens on the shoulders of others." His homily Sunday spelled out the spirit he wants church leaders to bring to the synod, as the meeting is called.

"Synod gatherings are not meant to discuss beautiful and clever ideas, or to see who is more intelligent," Francis said. Rather, it's an opportunity to "work generously with authentic freedom and humble creativity."

Conservatives want the synod to reinforce Vatican doctrine on sexual matters. Liberals hope the meeting will bring openness to the realities facing Catholic families.

Throughout his 1 ½-old papacy, Francis has stressed compassion and attention to the far reaches of his flock, including faithful who feel alienated from the institutional church. The synod provides an occasion to reflect on pastoral care for couples living together without marriage, gay unions and children in those families.

Francis also prayed for "peace for families and the entire world."

The synod ends Oct. 19.