There is a misconception that the Thor`s Hammer amulet arose purely or mainly out of some kind of heathen reaction against the encroachment of xtianity into Germanic northern Europe. One often sees this argument even in scholarly works, the authors of which should know better. However I would state that the popularity of the Hammer/Axe amulet increased as a result of the religious and cultural threat of xtianity but it does not have its origins in this.
As followers of my three blogs will know by now, I take the view that the Hammer of Thor is simply a stylistic and mythological development of the more ancient axe symbol. Indeed the club, axe and hammer are all closely related symbols of the Germanic Thunder God, *Thunaraz. The German Donar is more often than not associated with the Donarskeule or club as well as the better known hammer and amongst the Anglo-Saxons Thunor is known to have wielded a thunder axe:
"Se thunor hit thryscedh mid thaere fyrenan aecxe." (Dialogue of Solomon and Saturn)
"Thunor thrashes with his fiery axe."
"It is well known in England, and also in Germany, that no witch can step over a besom laid along the threshold of the house door on the inside. She will kick it or push it aside before she can enter your house, and by this token you may know her for what she is. An axe[Thor`s weapon] and a broom are laid crosswise on the innerside of the threshold over which the nurse has to step when she goes out with an infant to have it christened. This is done that the babe may be safe from all the devices of the powers of evil." (Curiosities of Indo-European Folklore, Walter Keating Kelly)
"As Indra used to milk the cloud cows and churn the milk lakes and fountains with the thunderbolt, so did Thor. The German god`s fiery weapon was often represented as an axe, and hence it is a customary thing with witches to draw milk from the handle of an axe stuck in a doorpost." (Kelly)
"Thor`s symbol in the Viking Age was both an axe and a hammer." (Scandinavian Mythology, HR Ellis Davidson)
Prior to the widespread practice of the wearing of the hammer there were many instances of Germanic people being buried with small votive axe pendants and of course axe-wielding Gods feature on Scandinavian rock art and on bronze figurines. The Celts, Balts and Slavs retained the axe as a symbol of their Thunder deity. At one time *Thunaraz was the primary deity of the Germanic peoples and even today His symbol is a sign that its wearer is an adherent to his native northern Gods.
"In northern Europe a cult of axes, in which axes unsuitable for practical use played an important role, is evident and is supported by archaeological finds dating later than the Neolithic Age. In the Bronze Age numerous rock carvings, and also the little bronze figure from Grevensvaenge, indicate a widespread cult of axes which should probably be understood as a fertility cult, as the phallic figures on the rock pictures suggest. Miniature axes as amulets are also documented since the late Iron Age and then again in the Viking Age.
"The great age of the Germanic axe-cult, the relationship with the Cretan axe-cult and the parallels to the lightning weapons of the non-Germanic gods, such as Indra`s and Hercules` clubs or Sucellos the Gaul`s hammer all suggest an Indo-Germanic origin of the various forms of the axe." (Dictionary of Northern Mythology, Rudolf Simek)