What I think the religion of the Celtic cross came from is probably the Pagan Druidic religion.
The religion was named after its founder, Zoroaster or Zarathustra.
There is no founder.
There was no founder of Hinduism.
Guru Nanak Dev Ji was the founder of Sikh Religion.
Traditional African religion is known as Paganism which is a collective name for all 'nature religions'. There is no sole founder of this religion.
there is no real founder of the Shinto religion
Celtic religions were practiced until the various Celtic peoples were Christianised in the early centuries A.D.
Duidism
The religion in which polyandry is allowed and incouraged is called a Celtic indigenous religion and other Celtic societies. Polygandry is when a woman is allowed to have 2 or more husbands at one time.
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.