The Presbyterian Church in Ireland is to hold a special meeting of its ruling body following the resignation of its leader earlier this week.
The Moderator, Dr Trevor Gribben, announced he was stepping down on Wednesday saying an internal audit had found major flaws in the church's safeguarding measures.
He will go at the end of the month - the first moderator to quit in the church's 185-year history.
He said some people had been harmed and he expected others to come forward.
It is understood the church began the internal governance review following the conviction of an individual for child sexual offences last year.
In a letter read to more than 500 congregations on both sides of the border this morning, the church said Dr Richard Murphy would take over as moderator in a fortnight.
He will immediately call a special meeting of the General Assembly later in the month.
In the letter the Acting Clerk of the General Assembly laid out the scale of the failings.
Reverend Dr David Allen said the church had not made mandatory referrals to statutory agencies when required. People had not been listened to when they voiced concerns, and convicted offenders who had returned to worship had not been properly monitored.
All of this happened between 2009 and 2022.
Read more: Presbyterian Church leadership under real pressure over safeguarding concerns
Dr Allen said the necessary safeguards had now been put in place.
"I must say now that these failures are inexcusable and no excuse is being made for them.
"On behalf of the Presbyterian Church I am deeply sorry. I'm sorry firstly, because people have been let down by us.
"Through past actions or not acting as we should, we have put people at risk and we know that some people have been harmed."
The Church announced on Wednesday that it was setting up a phone line and email address so that people who wished to raise concerns could come forward.
Dr Allen said if people did not want to approach the church, they should contact the police or victim-support services.
"Last Wednesday was the first time we were able to tell people that we had found certain things. That was just the start of a very difficult process. that's going to last for some time.
"We will not sweep this under the carpet, we will open ourselves to external review, and will cooperate fully.
"An external audit is already under way, designed by the Safeguarding Board for Northern Ireland, called Section 12.
"This audit will review our practices and processes, and will be submitted to the Safeguarding Board," said Dr Allen.
The PSNI is also in touch with the Presbyterian Church to ensure robust disclosure mechanisms are in place.
The Presbyterian Church has around 185,000 members in more than 500 congregations.
The majority of them worship in Northern Ireland, but there are significant concentrations in the border counties of the Republic of Ireland and elsewhere.
