The synod was not an attempt to “depart from the sacred patrimony of the truth received from the Fathers”, he said. But the Church must avoid becoming either “a rigid Church, which arms itself against the world and looks backward” or “a lukewarm Church, which surrenders to the fashions of the world”.

Church doors must be “open to all, all, all”, he added.

Conservative critics of the pope have become increasingly outspoken in advance of the synod, which is due to discuss topics including the role of women, acceptance of LGBT Catholics, and the impact of climate change on the poor.

Cardinal Raymond Burke, a Rome-based American who is one of the pope’s leading critics, has called for a defense against the “the poison of confusion, error and division” he feared the synod might introduce.

For the first time, women, including several nuns, will be allowed to vote, something that conservative women have contested, saying only bishops should have that right.

Two days before the synod started, five of the Church’s 242 cardinals disclosed they had sent a letter to the pope demanding clarifications on blessing for same-sex couples, the role of women and other issues.

The pope was joined in celebrating Wednesday’s Mass by most of the 21 new cardinals he promoted to the high rank on Saturday, a move that further cements his legacy. He has now appointed nearly three-quarters of the electors who will have the right to vote for his eventual successor.

Church leaders have been preparing for the month-long synod for the past two years, asking Catholics around the world to share their vision for the future of the Church.