Helgi the Lean Landnámabók, Part 2 Chapter XII
Helgi was very shifty in his faith; he believed in Christ, but made vows to Thor for seafaring and hardy deeds.
Source: Ari the Learned, "Helgi the Lean," The Northvegr Foundation, http://www.northvegr.org/lore/landnamabok/021.php, (2005).