The Bishop of Christchurch says the decision to demolish the cathedral was reached through prayer, great deliberation and with the utmost concern for safety.
Bishop Victoria Matthews says a number of options were considered before deciding to bring the walls down but the turning point was 23 December 2011, when a series of strong quakes rocked the city.
She says at that stage the Canterbury Earthquake Authority approached the church.
"CERA told us that our plans for making safe and retrieving, and then stepping back and making further decisions were no longer adequate."
The cathedral is now to be pulled down to two to three metres so taonga and heritage items can be retrieved.
She says heritage items will be removed and protected until plans for a new cathedral are made. This may be on the same site but it will not be a replica.
Bishop Matthews says the cathedral will be deconstructed with the utmost care and respect and without using bulldozers or wrecking balls.
A replica cathedral has been ruled out due to an estimated $100 million shortfall, while a new building incorporating some of the old would incur a shortfall of up to $50 million.
The building was seriously damaged during the earthquake in February last year and is cordoned off.
Deconstruction of unsafe sections has been carried out.
The building is 130 years old.
The Civic Trust says the church must halt the proposed demolition as once the building is partially deconstructed it will sound the death knell for the whole structure.
Neil Roberts of the city's Civic Trust is concerned that church leaders are moving too fast.
Mr Roberts says he fears that once the cathedral has been pulled down to a few metres, some may consider its heritage value to have been diminished.
And he says it may then be taken down completely.