Four people abducted and killed by republicans who suspected them of being British informers have still not been found decades later.
Archbishop of Armagh Eamon Martin said there was an urgent need to develop truth-telling mechanisms about the past violence.
He added: "There must be so many people walking around today who know in their hearts that the information that they have locked down inside them is capable of unlocking the uncertainty and grief of families."
Despite extensive searches, the bodies have never been found of four out of 16 people targeted by the IRA during the conflict.
The hunt has been led by the Independent Commission for the Location of Victims' Remains, established in 1999 by an agreement between the British and Irish governments to obtain information in strictest confidence that may lead to where the bodies are buried.
Archbishop Martin added: "For our part, we need to find a mechanism of truth and information retrieval which will allow more of these people to come forward so that many more families can be set free from the agony of waiting and wondering, 'why?'
"Even in the absence of a formal mechanism, I am confident that there are trustworthy people in society and in the churches who would be willing, and could be empowered and enabled, to accept and sensitively share information in this regard."
Joseph Lynskey, SAS-trained Captain Robert Nairac, Seamus Ruddy and Columba McVeigh remain missing.
The archbishop said a special Mass for the Disappeared at St Patrick's Cathedral in Armagh.
He expressed hope that someone would produce fresh or more precise information to help the Independent Commission with its search.
"There are people on all sides who carry secrets - memories of their own involvement in the deaths and injury of thousands of men, women and children.
"In some cases they pulled the trigger, planted the bomb, blindly followed orders or gave the command for death or punishment.
"In other cases they willingly drove a car, kept watch, spread fear, collected money or information, sheltered combatants, colluded or covered up, destroyed evidence or intimidated witnesses.
"These were awful, terrible times."
