it depends on what your definition of "religion" is.
we lack good sources because they were destroyed or written over by christian propagandists, but the sources we have suggest that the celts and germans were pretty similar to the very ancient greeks and that the reason is that the greeks ultimately came from the same origins as the celts and germans, before they migrated into greece. for that reason, the greek literature, in conjunction with very old literature from india, is the best information we have about what indigenous european culture was actually like.
we know that the europeans believed in democracy, which is why europe still has democracy; the christians were not democratic, and would have done away with it if they could have, but they couldn't and they failed. we know that germanic legal systems mostly followed processes that we still have remnants of in tort law, rather than civil law codes written by kings or churches, or the perpetual authority of the king.
we know the german and celtic gods were very similar to the greek gods, which is partly why the germans and celts let the greeks in to trade and largely avoided conflict with them but then fought the romans, years later. a lot of the cities in france were built by greeks and you can see the remnant overlap in the architecture, particularly in the cult of dionysus/diana, which is considered foundational to the spread of christianity.
it's no more clear that the germans or celts really believed in god, per se, than it is that the greeks did. when you have gods instead of god, you have magic and you have mysticism, animism and the sort. that is superstition, but it isn't clearly religion.
i think there's no clear answer to that question, and whether you should call indigenous european culture pagan or atheistic or superstitious or spiritual or animistic is not clear, but it is clear that europeans could never accept monotheistic belief or middle eastern legal codes, and never did.