I am looking for a fantasy series that was probably written in the fifties to the late sixties. I'm positive about most of the Author's name: Peter (S.) Tibblet (?). I know the last name began with T and there was a double letter -- almost certainly -bb- of my memory is correct.
The trilogy was somewhat more like three separated books with a common theme of one religion taking over from the last. There was, in the third book, a Druid and his apprentice, who travelled to Britain/Avalon. There the apprentice fell in love with one of the druid's equivalent priestess-in-training on the Isle, which was Priestess-based Mother Goddess religion.
There were two ways of representing the same form. In the Druid's case, it was a magic square. However, if one took the lines of a magic square -- in the case of Mercury, that is the three-by-three magic square:
| 4 9 2 |
| 3 5 7 |
| 8 1 6 |
If you took an ordinary square, numbered as your ordinary telephone keypad, and connected rows of dots, you got a representation of what the Goddess-based religion used: a V and an upside-down V with a diagonal line through both of them. (This is also the sigil for Mercury and a lot of other gods of communication.) Similarly with the 4-,5-,6-,7-, and 8- magic squares (though there was a "trick" to solving the Sun's square). This thrilled the apprentice since it showed they were worshipping the same deities, though they had different representations of them.
At the end of the third book, the old Druid went to the astral equivalent of their Temple and changed the representation of the religion representing the Light to the symbol of the "white god's" new religion, the Cross. Shortly afterwards, either barbarians or Romans invaded the temple and killed all of the priests and priestesses and all trainees and apprentices. But they were too late; the defenders of the Light was now Christianity.