Christianity came to Norway in the 9th and 10th centuries, arriving from the south (via Denmark) and from the British Isles. The influence from the British Isles is particularly important, and the first Norwegian kings were baptised in England.

Even though Norway was influenced by Christianity for a few centuries throughout the Viking period, it wasn’t until Olav Haraldsson that Christianity became permanent. Also, Olav Haraldsson became St Olav – whose casket was placed at Nidaros Cathedral. 

Olav Haraldsson was born in 995 and at the age of 12 set off to England on a Viking raid. During his time in England, he discovered Christianity and was baptised in 1014. He then returned to Norway with a mission: convert the Norwegians. He crowned himself King and then travelled around the country forcing Christianity on the pagans, or he threatened them with death. Eventually his strict reign created enemies, and he was forced to flee Norway in 1028. He came back a year later in an attempt to retake the throne, but at Stiklestad (a region north of Trondheim), his opponents lay in waiting with a huge army. After an intense battle, on the 29th of July 1030, Olav was killed. 

His body was transferred to Trondheim and buried in a sandbank. It is believed to have been roughly where Nidaros Cathedral’s high altar is today.

Rumours quickly spread that miracles had occurred by the kings body, and that Olav was holy. Around one year after his burial, the coffin was removed and opened. The kings body appeared unchanged as if he had just died, and his hair and nails had grown. 

Olav was proclaimed a saint and his coffin was placed on the high altar at St. Clements Church. A wooden chapel was built on the place where he had been buried, and a spring arose from there that had healing properties. Countless miracles were happening and the casked of St. Olav became an important pilgrimage site.