Arianism was so far from being dead in 325, that it spread among the Goths, so that the first king of Spain to be a Catholic did not become a Catholic until 586: 261 years after Nicea, & 268 after Arius wrote to Alexander of Alexandria.
You know, I was pretty surprised when I first learned this (reading St. Isidore’s history of the Goth’s and other groups). I always assumed Nicea ended that issue–but many nations were completely Arian for centuries after (as you point out). They would often shift back and forth between Arianism and orthodoxy, depending on the preference of the King.
Of course, we can also talk of the massive expansion of Protestantism and all its branches over the last 450 years–and the growth of Mormonism in more recent times