Some abuse victims have called for the interim advocate for survivors of historical institutional abuse to resign after he assisted in a Catholic church service on Sunday.
Brendan McAllister was appointed to the role in July 2019.
Victims' campaigner Margaret McGuckin said it was a "conflict of interests".
Mr McAllister has said he was a candidate for ordination as a deacon in the Catholic Church in 2021.
He robed in vestments to assist in the liturgy at St Peter's church in Warrenpoint, County Down.
Ms McGuckin, from Savia (Survivors and Victims of Institutional Abuse), claimed that it was a "big conflict of interest" since many survivors in the group are victims of clerical abuse in the Catholic Church.
"We knew last year he would be ordained as a Catholic Church deacon.
"We let it be as he wasn't going to be ordained until after his role was over," she said.
"As a Christian everyone has an entitlement to worship. I'm a Christian myself but this is different," she added.
"How can he be impartial? How can his loyalties lie with abuse victims, the majority of whom have been abused by members of the Catholic Church?"
She said victims would be "re-traumatised" as "many of our members were abused by priests dressed in this regalia".
"You cannot have one foot in and one foot out," she said.
Mr McAllister told the BBC the leaders of all five HIA victims groups were aware of his church role and had "previously expressed their good wishes".
"From the time of my appointment as interim advocate, senior officials of the Executive Office have been aware that I have been preparing for future ministry in the Catholic Church," he added.
'An amazing advocate'
Jon McCourt from another victim's group, Survivors North West, defended Mr McAllister and dismissed claims of a conflict of interest.
Mr McCourt said he had "no concerns" about his role which in "no way impacts his ability to be the interim victims' commissioner".
"How he decides to further his relationship with God is his business," he told BBC's Talkback programme.
"He has been an amazing advocate," he said.
"There may be a perception of a conflict of interest from others, but Mr McAllister has not said there is," he said.
"He has enough integrity that if he thought there was one he would say there was," he added.
In May, Mr McAllister apologised after the identities of 250 survivors of historical institutional abuse were revealed in an emailing error.
A newsletter was sent without recipients' names being anonymised.
The email was signed by a staff member, but sent on behalf of Mr McAllister.
He said he took full responsibility for the error and has referred the matter to the information commissioner.
Mr McAllister was appointed by the head of the civil service, David Sterling, while the Assembly and Executive were collapsed.
He is a former social worker with the probation service, a former victims' commissioner, and has had a distinguished international career as a peace mediator.