Reconstructed Proto-Germanic vocabulary underlies much of what can be said with any confidence about pre-Christian Germanic religion, since the alternative, medieval Christian sources such as Snorri Sturluson's Prose Edda, were written centuries after conversion by authors working at a real remove from the beliefs they described.
The historical runic alphabet, the Elder Futhark, is itself named through Proto-Germanic: its first six letters spell out futhark, and its individual rune names, reconstructed forms such as fehu, cattle or wealth, and wunjo, joy, are Proto-Germanic words rather than later inventions, giving each rune a claim to genuine period meaning rather than only a later reinterpretation.
Several major Norse deity names are likewise reconstructed to Proto-Germanic roots shared across the wider Germanic world rather than invented fresh in Scandinavia, most notably *Wodanaz, ancestor of both Norse Odin and Old English Woden, whose wide geographic spread, attested in place names and the weekday name Wednesday across Germanic Europe, argues for a cult considerably older and more widespread than the Old Norse sources alone would suggest. Because so much of Germanic pre-Christian religion is known only through later, Christianized retellings, this linguistic layer, words reconstructed rather than texts transmitted, is often the most direct surviving evidence of what the older tradition actually called its own gods and rites.