What I think the religion of the Celtic cross came from is probably the Pagan Druidic religion.
religion
Celtic religions were practiced until the various Celtic peoples were Christianised in the early centuries A.D.
education and religion
Duidism
Celtic religion started before Jesus had even come, they have been around forever, but the longer they go on the more they change and are forgotten.
The "Celts" of the English Isles were known for their Druid religion ( a pagan or Wicca like religion) but there ws also a large Christian grouping. The Celtic Cross was designed by these early Celtic Christians.
A Druid
Celtic Football club has never discriminated against religion or race.... we leave that to Rangers
an emotional and revivalistic style of religion.
Ray Simpson has written: 'Exploring Celtic spirituality' -- subject(s): Celtic Church, Celts, Religion, Spirituality
Edain McCoy has written: 'Witta' -- subject(s): Celtic Magic, Magic, Celtic, Paganism, Witchcraft 'Celtic Myth & Magick' -- subject(s): Celtic Gods, Celtic Magic, Celtic Mythology, Gods, Celtic, Magic, Celtic, Mythology, Celtic 'Inside A Witches' Coven (Llewellyn's Modern Witchcraft Series)' 'Making Magic for Witches and Pagans' 'Spellworking For Covens' 'Bewitchments' 'A Guide to Astral Projection' 'Celtic women's spirituality' -- subject(s): Celtic Goddesses, Celtic Magic, Celtic Mythology, Goddess religion, Goddesses, Celtic, Magic, Celtic, Mythology, Celtic, Religious life, Women 'A witch's guide to faery folk' -- subject(s): Fairies 'Inside a witches' coven' -- subject(s): Covens, Witchcraft 'How To Do Automatic Writing (Llewellyn's \\' 'Ostara' 'Magick & rituals of the moon' -- subject(s): Miscellanea, Goddess religion, Rituals, Magic, Neopaganism