St Mary's Church in Llanfair Kilgeddin, between Abergavenny and Raglan, was targeted between 2 and 3 August.
An oak door, valued at about Â£1,000, was damaged, along with four oil lamps dating back 300 years. Metal organ pipes and a brass plaque were stolen.
Matthew Saunders, director of Friends of Friendless Churches, said: "They left it looking like a bomb landed."
Gwent Police is investigating the theft and Mr Saunders said he thought thieves were targeting metal inside the building.
"It's completely absurd because the metal is worth practically nothing," he said.
"We're very upset by it, but we will not be deterred from opening the church."
Friends of Friendless Churches saved St Mary's from closure when it took over in the late 1980s. The building is medieval in origin.
Mr Saunders said the total value of the theft and damage was not fully known, adding: "It's both petty and monstrously disrespectful."
A decision on what to do about the missing pipes from the mid-19th Century organ will be taken when the church committee meets in October.
"Monstrously disrespectful" thieves have wrecked and ransacked a church in Monmouthshire.