Ulster Unionist MLA Robbie Butler has said he is surprised and disappointed at social media criticism of his decision to meet the Pope.
The Lagan Valley representative will attend Pope Francis's address in Dublin Castle tomorrow.
Mr Butler, who is a born-again Christian, posted a video on Facebook saying he was delighted to be representing his party during the first papal visit to Ireland in almost 40 years.
While many people praised him, others accused him of abandoning Protestantism and unionism.
Former South Down UUP MLA Harold McKee said on the papal visit that Mr Butler "doesn't represent me as a member of the UUP". But the wife of suspended DUP MP Ian Paisley offered her support.
Fiona Paisley said: "My head is in my hands after reading some of the replies to your post Robbie, each to their own view. Always do what you think is right in your heart."
Mr Butler, who is a deacon in Maghaberry's Elim Church, is deputising for his party leader at the event because Robin Swann will be taking part in a Royal Black Preceptory parade in Larne.
In his video, the MLA said the papal visit was of major significance to hundreds of thousands of Catholics on both sides of the border.
Defending his decision to attend the Dublin event, he said: "The Ulster Unionist Party is a political party, not a religious party."
He said he was looking forward to hearing what Pope Francis said and hoped he would address the challenges faced by the church on clerical sexual abuse.
While some on Facebook praised Mr Butler's "gesture towards reconciliation", others described the Catholic Church as "evil" and "a cancer on all that's good". Some posters referred to "the Papists" and "thousands of Protestants martyred by the Papist Church".
One poster commented: "Very sorry to see as a Protestant you are looking for hope from the anti-Christ in Rome.
"I urge you in God's name to take a stand for Christ and wash your hands of the unclean filthy thing." Another said the UUP had lost its "backbone as well as voters".
Mr Butler said: "I respect that some people have a different opinion to me on going to meet the Pope but I am still surprised and disappointed by some of the comments.
"Views have resurfaced which I had hoped belong to the past.
"I am confident enough in both my faith and my brand of unionism to attend the Dublin Castle event.
"I feel very strongly about the need to be respectful, open and embracing and to treat my Catholic neighbours as I'd like to be treated myself."
The UUP MLA said unionists had to reach out to the entire community. "As head of the Church of England, Queen Elizabeth has met five popes during her reign. She spent five hours in the Vatican with Pope Francis in 2014 so surely it is acceptable for a unionist to attend one short event in Dublin Castle?" he said.
"Unionism is not and should not be restricted to one religion. It must reach out to all sections of the community.
"As a fireman, I worked with a mixed crew and we served diverse communities. Religion didn't come into my work then and it doesn't now. It's important that unionists build bridges whenever the opportunity arises."
Regarding the comments by his party colleague Mr McKee, Mr Butler said: "Harold is a very good friend of mine. We have a difference of opinion on this issue but that won't prevent us continuing to be friends."