The century-old cathedral in Xuan Truong District, 78 meters (256 feet) long and 15 meters tall with 35-meter towers, would be brought down in a few more days, Priest Joseph Nguyen Duc Giang, general representative of the Bui Chu diocese, said.
Giang said the demolition plan is decided by the church's Bishop Thomas Vu Dinh Hieu.
"The new church will be built exactly like the old one and all the furniture and ornaments in the old church will be installed in the new one."
Vu The Du, a parishioner, said he has been informed by the bishop that the demolition and rebuilding would start in late February.
When the work is going on, the service will be organized at a makeshift church which has already been set up nearby.
Do The Thong, another parishioner, was unhappy that "a church which has witnessed history of the diocese will be destroyed" but admitted it is necessary since the building is rundown and even threatens the safety of churchgoers, especially during rains.
He believed the new church would be just like the old one "with just a few changes to make it suitable for modern times and serving more people."
Built in 1885 by Spanish Bishop Wenceslao Oñate, the cathedral was in the news last year when the diocese announced plans to rebuild it.
But many, including architects, protested against this, given its unique historical, artistic and cultural status.
Twenty five architects wrote to Prime Minister Nguyen Xuan Phuc, Minister of Culture, Sports and Tourism Nguyen Ngoc Thien and Nam Dinh Province Chairman Pham Dinh Nghi asking them to stop the demolition.
The cathedral was "an architectural, artistic and cultural heritage that our ancestors put much work into to create, a unique architectural work found nowhere else in Vietnam and of a piece with other national cultural heritages," the letter said.
"The structure not only uses European architecture but also combines Vietnamese elements, details and materials to create a unique artwork demonstrating an east-west cultural fusion."
They also said they had inspected the structure and found it was only slightly damaged, its frame was still good and could last long if reinforced. They pleaded with the PM to stop its demolition until the National Heritage Council made a thorough assessment.
A delegation from the culture ministry, including officials from the cultural heritage department, visited the place in May last year and came to the conclusion that the church is indeed badly degraded, many aspects such as the door and dome are cracked, and one of its two towers is no longer standing straight.
Bui Chu parishioners said the old cathedral had great significance for them, but its condition was so bad that it was dangerous.
Plaster from the ceiling falls down frequently, sometimes injuring churchgoers, once so badly that a person had to be hospitalized, they said.
The electricity system is also unsafe and threatens to cause short circuits and fires, they added.