Pope Francis says a creative, humble approach is needed as the Vatican tries to understand 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 much of their flock, while considering themselves Catholic, defy church teaching on family matters. The most contentious issues include bans on artificial birth control and on Communion for divorced faithful who remarry.

Francis told the churchmen not to "discuss beautiful and clear ideas" but use "authentic freedom and humble creativity" in debate.

Conservatives want to reinforce Vatican doctrine. Liberals hope the meeting will bring openness to Catholic families' realities.