Pope Benedict, opening the first major Church meeting since his election, said yesterday that trying to keep God out of public life was "not tolerance but hypocrisy".
The Pope, during a solemn Mass in St Peter's Basilica opening a synod of more than 250 bishops from 118 countries, also said that too many Catholic lives could be compared to "vinegar rather than wine" because of the indifference to God.
He homed in on concerns dear to him since his election on April 19 - the health of the faith of individual Catholics and the entire Church and God's place in the public life.
"The type of tolerance which permits God as a private opinion but refuses to allow him in the public arena, is, in the reality of the world and our life, not tolerance but hypocrisy," he said in the homily before the bishops.
"There can be no justice where man makes himself the only master of the world and of himself," the Pope said during the Mass attended by thousands of people.
The official theme of the synod is the sacrament of the Eucharist, which Catholics believe is Christ's body and blood.
But discussion is expected on issues such as the shortage of priests, the decline in Mass attendance and whether Catholics who back abortion rights should be able to receive Communion.
The US Catholic community was divided last year over whether they should support presidential candidate John Kerry, himself a Catholic who supported abortion rights.
Some Catholics say they personally would not have an abortion but, in pluralistic societies such as the United States, feel obliged to support a woman's right to choose.
But the synod's working document says Catholics cannot back politicians who openly support abortion.
There also have been legal conflicts in the United States and Italy about whether religious symbols such as crucifixes and the tablets of the Ten Commandments can stay in public places.
The three-week-long synod will offer advice to the Pope, who is expected to attend all or most of the sessions, giving him a chance to meet many of the bishops for the first time.
In another part of the homily, the Pope elaborated on the Biblical story of the vineyard which produced bad grapes.
"Is our Christian life often perhaps much more vinegar than wine? Is it self-commiseration, conflict, indifference?"
Since his election the Pope, who last August in Germany said there was much to criticise in the Church, has urged Catholics to take their faith much more seriously.
The synod's working document says that an increasingly secularised society had weakened the sense of mystery in the sacrament of Communion.
"I hope the synod leads to a deeper appreciation of the Eucharist and what it means for us as a sacrament. We have to make sure we are faithful to that celebration on Sunday despite the cultural pressures against it," said Bishop William Skylstad, president of the US Conference of Catholic Bishops.
Bishope Skylstad told Reuters he expected some talk on the shortage of priests but was not expecting any substantial discussion about the Church rule on priestly celibacy. Some groups believe the shortage could be eased if priests are allowed to marry.
The Pope had invited four bishops from China but the communist government has not yet given them permission to leave.