Iceland was Christianized in the year 1000 AD, when Christianity became the religion by law, according to the Heimskringla. In Icelandic, this event is known as the kristnitaka (literally, "the taking of Christianity").
The earliest Christian observance in the country in all likelihood began with the arrival of the first settlers during the settlement of Iceland in the 9th and 10th centuries AD. Some of them were from the British Isles and had adopted Christianity through their contact with the Irish. However, the vast majority of the initial settlers were pagan, worshipping the Æsir (the Norse gods), and organized Christian observance probably died out within a generation or so.
Iceland's adoption of Christianity is traditionally ascribed to the year 1000 (although some historians would place it in the year 999).
The major sources for the events preceding the adoption of Christianity are Ari Thorgilsson's Book of the Icelanders, the Icelandic family sagas and Church writings about the first bishops and preachers. Ari's account of the events surrounding the conversion seems to be reliable; although he was born 67 years after the conversion, he cites first-hand sources.
Beginning in 980, Iceland was visited by several missionaries. The first of these seems to have been an Icelander returning from abroad, one Thorvald Konradsson. Accompanying Thorvald was a German bishop named Fridrek, about whom little is known. Thorvaldur's attempt to convert Icelanders met with limited success. He was the subject of ridicule and was eventually forced to flee the country after a conflict in which two men were killed by Thorvald.
When Olaf Tryggvason ascended the throne of Norway, the effort to Christianize Iceland intensified. King Olaf sent an Icelander named Stefnir Thorgilsson back to his homeland to convert his fellow countrymen. Stefnir violently destroyed sanctuaries and images of the heathen gods – this made him so unpopular that he was eventually declared an outlaw. After Stefnir's failure, Olaf sent a priest named Thangbrand. Thangbrand was an experienced missionary, having proselytized in Norway and the Faroe Islands. His mission in Iceland from c. 997–999 was only partly successful. He managed to convert several prominent Icelandic chieftains, but killed two or three men in the process. Thangbrand returned to Norway in 999 and reported his failure to King Olaf, who immediately adopted a more aggressive stance towards the Icelanders. He refused Icelandic seafarers access to Norwegian ports and took as hostages several Icelanders then dwelling in Norway. This cut off all trade between Iceland and its main trading partner. Some of the hostages taken by King Olaf were the sons of prominent Icelandic chieftains, whom he threatened to kill unless the Icelanders accepted Christianity. In the 11th century, three Armenian bishops, Petros, Abraham and Stephannos are recorded by Icelandic sources as Christian missionaries in Iceland. Their presence has been explained in terms of the service of King Harald Hardrada of Norway (c.1047–1066) as a Varangian in Constantinople, where he had met Armenians serving in the Byzantine Imperial Army.