This is a very long, but Christianity by missionaries, and the possibilities for the powerful meant spread rapidly in the Dark Ages.
Furthermore, thanks to religious syncretism, traditions and pagan cults were assimilated into the Christian religion, which had the most intimate to each person with his religion also became Christian.
Ireland and the Picts, developed a particular christianity, Celtic Christianity, with very specific characteristics, but it was Christianity. Moreover, Irish missionaries were among the most prolific knew