Shortly after Olaf Tryggvason became King of Norway he decreed that the old faith should be discarded and replaced with Christianity. His decree extended also to the islands of Shetland, Orkney, and Faroe.
When news of Norway's conversion reached Iceland, it was received by many with great anger. "It is monstrous," they said, "to forsake our ancient beliefs."
But Njal, a respected leader known for his ability to foresee the future, replied, "I support the new faith. I believe that Christianity is a better religion than our old one. Those who accept it will be happy."
King Olaf of Norway made preparations to convert Iceland. To this end he sent Thangbrand, son of Count Willibald of Saxony, on a mission to Iceland. Accompanied by the Icelander Gudleif Arason, a renowned warrior, Thangbrand set sail for Iceland aboard a ship named The Bison. They landed in the eastern fjords at a place called Gautavik. They were met there with hostility. Most Icelanders of that district refused to sell them provisions or to trade with them, but Hall of Sida opened his house to them and offered them hospitality.
One morning – it was an important feast day – Thangbrand sang mass, and Hall asked him, "In whose honor is this ceremony?"
"In honor of the angel Michael," answered Thangbrand.
"Does he have great power?" asked Hall.
"Yes, he has great power," replied Thangbrand, then continued, "The angel Michael will be your friend and guardian if you will promise yourself to him in God's name."
"That I shall do," promised Hall, and a short time afterward he and his entire household were baptized.
The next spring Thangbrand, accompanied by the warrior Gudleif and the new convert Hall, preached Christianity throughout the land.
At Stafafell a farmer named Thorkel challenged Thangbrand to a duel. Thangbrand accepted the challenge and went to battle using a crucifix for a shield. Victory went to Thangbrand, and he killed Thorkel. Then Thangbrand and his companions continued to travel from district to district, converting many prominent families to the new faith.
Frightened at the Christians' success, the heathens at Kerlingardale hired a sorcerer named Hedin to kill their leader Thangbrand. The sorcerer accordingly went to the Arnarstakk Heath where he conducted a great sacrifice. At the time Thangbrand was riding westwards, and the ground suddenly opened up beneath his horse. The horse disappeared into the earth, but Thangbrand miraculously pulled himself to safety.
Thangbrand's companion Gudleif searched out Hedin the sorcerer and killed him with a spear.
Next they preached the new faith at Fljotshlid, where they met great opposition from Vetrlidi the Poet, so they killed him. From there they went to Bergthorsknoll, where Njal and his entire household were baptized.
At Grimsness Thorvald the Ailing had gathered a large band of Icelanders against the missionaries. They attempted to ambush the Christians, but one of their number warned the missionary group. Forewarned, Thangbrand, Gudleif, and their followers rushed the would-be ambushers. Thangbrand threw a spear through Thorvald, then Gudleif cut off his arm, and Thorvald died.
The missionaries then rode on to the Althing. Thorvald's kinsmen had assembled there to avenge the death of their relative, but Njal kept the two warring groups apart.
At the Althing Hjalti Skeggjason, a new convert to Christianity, composed a poem that stated in verse:
I dare mock the gods.
I believe that Freyja is a bitch,
And that Odin in a dog,
Or else the other way around.
Later that summer Thangbrand's ship, The Bison, was wrecked. It is not stated whether Thangbrand himself was aboard at the time, but in any event, he continued his missionary activities. This shipwreck caused some heathens to claim that their god Thor's giant-killing hammer had struck dead the Christian bison, thus proving that Christ was powerless to confront a challenge from Thor.
To this Thangbrand replied, "Thor lives only at the will of the Christian God. Without my God's permission, Thor would be nothing but dust and ashes."
At Hagi a man named Gest Oddleifsson held a feast for Thangbrand and sixty of his followers. Some two hundred heathens had gathered there as well. They expected to be joined by a berserk named Otrygg. It was said that Otrygg feared neither fire nor sword.
Thangbrand declared that he would use the berserk to test the power of Christianity over that of the old religion. "We shall light three fires," he proposed. "I shall bless the first one, you heathens shall bless the second one, and the third one shall remain without a blessing. If the berserk walks through your fire unharmed, but is afraid of my fire, then you must accept Christianity."
Gest, the leader of the heathens, believing that the fearless berserk would walk through all the fires, accepted this challenge.