Gilles Mallet, who is 69 and has been in custody since the fire, appeared virtually in Bathurst provincial court Monday before Judge Éric P. Sonier.
The guilty plea came after a hearing to determine Mallet's fitness to stand trial.
A publication ban prohibits the reporting of the details of the court proceedings in this case.
Mallet was arrested only hours after fire levelled the Saint-Simon church overnight on Aug. 23.
The church was built in the 1910s and was believed to be the largest wooden church left in the province. Saint-Simon is now part of the Municipality of Caraquet.
Firefighters got to the fire around 4 a.m. on Aug. 24 but could not contain the flames.
Fire teams from three municipalities continued to fight the fire until 10 a.m., but the church was a total loss.
No one was in the building when it burned.
RCMP said at the time that the Office of the Fire Marshal concluded the fire had been intentionally set.
Mallet remains in custody and is to appear in court on Dec. 14 for sentencing.
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