Peter Saunders said he's "confident" the Vatican will change its mind and remove Juan Barros as the Bishop of Osorno in southern Chile, because of allegations he covered up the child abuse of another colleague.
Mr Saunders was speaking after he and three others from the Pontifical Commission for the Protection of Minors flew to Rome to have an urgent meeting with its President, Cardinal Sean O'Malley.
They voiced extreme concerns about Juan Barros' appointment as bishop in January, given the allegations against him.
Cardinal Sean, he said, agreed with the group's concerns and promised to bring the matter directly to the pontiff either on Monday or today. This is despite the Vatican publicly supporting Juan Barro's appointment in March.
Peter Saunders told Premier: "I think they have potentially underestimated the... huge numbers of people in Chile at the moment who are extremely unhappy with this appointment, and this is not just the victims and survivors of Karadima, this is many other people: the congregations, the priests, the politicians.
"They seem to be united in their opposition to the appointment of this man [Barros], and I would expect and am confident that there would be change soon."
"I think it's battling against evil to be quite honest, because I consider the abuse of children to be an act of evil. It is tempting to throw in the towel, but I think this is a war that we are in...
"Many people in many positions of power... have got away with murder for far too long, and I think the tide is now slowly turning...
"I can't think of a bigger institution or a more powerful person or influential person at this moment in time than Pope Francis. He has a unique opportunity now to make things happen."
Juan Barros is accused of covering up the child abuse committed by Fernando Karadima, another Chilean priest.
Fernando Karadima was convicted of child abuse in 2011 and forced to retire, however Bishop Juan denies hiding Karadima's abuse.
Listen to Peter Saunders speaking to Premier's Aaron James: