Offenders smashed through a stained-glass window to get inside St Mary and St Martin's Church on Sheffield Road, Blyth.
Several artefacts were damaged during the attack at the Grade I listed church.
This includes an effigy of a knight dating back to the 13th century, which was damaged after bricks were dropped on it.
A knife was also stabbed into the wooden door leading to the church bell tower.
Graffiti was scrawled on the walls, cash was taken and several items of religious literature were also damaged.
The break-in took place at some point between 7pm on Thursday, February 20, and 2pm on Sunday, February 23.
The church's origins date back to 1088, with the site being one of the oldest examples of Norman architecture in the country.
The incident is now being investigated as a "heritage crime". These crimes are defined by Historic England as any offence which harms the value of heritage assets and their settings.
Officers have asked for the public's help and are looking to speak to anyone holding information about or CCTV footage of the incident.
Inspector Rob Harrison, district commander for Bassetlaw, said: "This was an appalling attack on a beautiful and historic building which has understandably upset people in the village.
"We are determined to trace those responsible and believe the public could have an important part to play. The church and its grounds are frequently visited by people and we’d ask if you passed by and noticed anything which could assist our investigation to please get in touch.