Pope Francis opened a divisive meeting of the world's bishops on family issues by forcefully asserting that marriage is an indissoluble bond between man and woman. But he says the church must "seek out and care for hurting couples with the balm of acceptance and mercy."

In a solemn Mass in St. Peter's Basilica on Sunday, Francis dove head-on into the most pressing issue confronting the meeting of 270 bishops: How to better minister to Catholic families experiencing separation, divorce and other problems.

He said: "The Church must search out these persons, welcome and accompany them, for a church with closed doors betrays herself and her mission and instead of being a bridge, becomes a roadblock."

One of the major debates at the synod is whether divorced and civilly remarried Catholics can receive Communion.

Few Vatican meetings have enjoyed as controversial a run-up as the three-week gathering of bishops. There have been allegations of manipulation and coercion. Secret caucuses to plot strategy, and on the eve of the synod, a Vatican monsignor outing himself, urging the pope to hear his voice, and denouncing widespread homophobia in the church.

The church has been dogged by deepening divisions between conservatives and progressives over issues of ministering to today's Catholic families in all their shapes and colors, including gays, the divorced and others in "irregular" family situations.