From 5th October, senior Catholic leaders will discuss relationships within the Church and several have already spoken about the need to change rules regarding divorced people.
Currently, a divorcee who has remarried is not allowed to take communion, but some have said that there should be certain exceptions to this rule.
Forty-eight people – mostly conservative intellectuals and ministers – have written an open letter to the Synod asking for the Church's official stance on divorce to remain firm.
They include American evangelical leader Rick Warren, former president of the Italian Senate, Marcello Pera, and Harvard Law professor Mary Ann Glendon.
"The consequences of divorce and cohabitation for children and adults are many and diverse – from poverty and lower educational achievement to poorer physical health; from lower marital commitment in adulthood to earlier death," the letter said.
It added: "May we humbly suggest that in the context of marriage and family life your words are a call to personal responsibility, not only for our own spouses and children, but for the marriages of those God has put by our side: our relatives and friends, those in our churches and in our schools."
The letter also outlines ways to stop married couples from choosing to get divorced.
The Synod will meet in Rome from 5th October until 19 October.