Answer based on context:

While attending to his pastoral duties, Laestadius continued his interest in botany and authored a number of articles on plant life in Lapland.  Because of the wide recognition for his knowledge of botany and the Sami, the French Admiralty invited Laestadius to participate in the La Recherche Expedition to Samiland of 1838-40.  As an expedition member, Laestadius served as field guide for the islands and the interior of Northern Norway and Sweden, studying both plant life and the culture of the Sami inhabitants. During the expedition, Laestadius at the request of the organizers began his manuscript.  Eventually published for the first time more than 150 years later Fragments of Lappish Mythology provides a snapshot of Sami traditional religious beliefs that by the 1830s were passing into history due to the Church of Sweden's Christianization mandate in full swing at that time.  However, Laestadius did not finish the manuscript until long afterward.  Then the completed work was lost for many years.  Due to these and other reasons, the manuscript was not published until 1997, over 150 years after the expedition. For his participation in the La Recherche Expedition, Laestadius was awarded the Medal of Honor of the Legion of Honor of France after 1841.  He was the first Scandinavian to receive this honor.

The Church of Sweden was causing what culture's beliefs to die off?
Sami