The run-up was dominated by a row over a Vatican priest who on Saturday announced he was in a gay relationship.
Poland-born Krzysztof Charamsa said he wanted to challenge the Church's "backward" attitude to homosexuality.
He was later dismissed from his post at the Vatican's office in charge of guarding Roman Catholic doctrine.
A Vatican spokesman said Monsignor Charamsa's decision to give interviews on the eve of the synod was "grave and irresponsible" and would put Pope Francis under "undue media pressure".
In an interview with the Corriere Della Sera newspaper, the 43-year-old priest said: "It's time the Church opened its eyes and realised that offering gay believers total abstinence from a life of love is inhuman."
The controversy has set the scene for what some fear could be a fractious three weeks, says BBC religious affairs correspondent Caroline Wyatt.
The Synod - a defining moment for Pope Francis
Almost 300 Church leaders - and some lay people - will be discussing such issues as the treatment of Catholics who are gay, and how to approach couples who live together without being married or wish to take communion after being divorced.
Analysis: Caroline Wyatt, BBC News
It is the most highly-anticipated meeting of the leaders of the Roman Catholic church since the Second Vatican council, some 50 years ago.
In his homily today, the Pope spoke of the importance of the family - and perhaps rather pointedly, of the love between a man and a woman. He also said that the wealth of the developed world hadn't led to greater happiness, but to loneliness, and higher rates of abortion and divorce.
Many campaigners and more liberal Catholics would like to see a change in Catholic doctrine on issues ranging from homosexuality to contraception.
But while some bishops here do want reform, others remain fiercely opposed. In the end, though, the synod is not a democracy. It will be up to the Pope to decide what - if anything - changes, and how the Church applies its teaching to its sometimes errant flock.
The Pope has called for a more understanding attitude on sexual issues.
After his election in 2013, Pope Francis reaffirmed the Roman Catholic Church's position that homosexual acts were sinful, but said homosexual orientation was not.
"If a person is gay and seeks God and has good will, who am I to judge?" he said.
The issue of homosexuality was also highlighted during the Pope's visit to the US last week.
He had a private meeting with a gay former student of his and his boyfriend at the Vatican mission in Washington.
The Pope also met Kim Davis, a Kentucky local official who recently gained attention for refusing to issue marriage licences to same-sex couples.

Low numbers of staff with the right specialist training at Stafford's County Hospital forced the closure of A&E to patients under 18, on 25 August.
The hospital trust said it was looking at putting in a service to deal with minor injuries.
Plans for a full range of services have been submitted for scrutiny.
The A&E department is open to adults.
The hospital, under its former guise of Stafford Hospital, was the subject of a public inquiry after a higher than expected number of deaths between 2005 to 2008.
More on this and other stories from Stoke-on-Trent and Staffordshire
Robert Courteney-Harris, chief executive of the University Hospitals of North Midlands NHS Trust, said staff understood concerns over the decision to suspend children's emergency services.
"We recognise that a small number of patients and their families have now had to travel further for non-urgent treatment and we are very sorry for this," he said.
"However, ignoring external advice about the safety of our services could put some children's lives at risk, something the trust is simply not prepared to do."
The Royal College of Paediatrics and Child Health has been asked to carry out a service review and to make recommendations about how it can be made safe. The process is expected to last 10 weeks, he said.
In the meantime, staff are looking at ways of setting up a minor injuries service to deal with broken bones and sprains.
University Hospitals of North Midlands NHS Trust took over the County Hospital in October last year, after the previous trust was declared clinically and financially unviable.
The now-defunct Mid Staffordshire NHS Foundation Trust was fined Â£500,000 last year after admitting four charges over the deaths of elderly people, between 2005 and 2014.
Pope Francis has celebrated Mass at St Peter's Basilica in the Vatican, at the start of a synod of Roman Catholic bishops focusing on family issues.
[NEXT_CONCEPT]
An A&E unit hit by staff shortages should have an emergency service for children with minor injuries within a few weeks, doctors hope.