Turkey was eaten in Plymouth because it was a common and tasty bird, not for religious meaning. Interestingly there was actually another Thanksgiving two years earlier in Virginia that people ignore. How turkey became accepted as a kosher bird is itself an interesting story.
But the first national Thanksgiving was in 1789, declared by the famous philo-Semite President George Washington. It was not Christian (although Washington himself was) and was enthusiastically embraced by the Jews of New York.