The word trinity comes from the Latin 'trinas' and 'unitas' meaning 'three that unite'.
And another from the same book p32
According to the dictionary of symbols, the triquetra or triqueta predates Christianity and was likely a Celtic symbol of the Goddess, and in the North, a symbol of the god Odin....
Triplicities were common symbols in Celtic myth and legend, one of the possible reasons Christian beliefs were so easily adopted by the Celtic people. The triqueta makes an ideal Christian symbol. It is a perfect representation of the concept of “three in one” in Christian trinity beliefs, and incorporates another popular Christian symbol, the fish, in its original form of the vesica pisces. It is sometimes enclosed within a circle to emphasize the unity aspect. http://symboldictionary.net/?p=159
With this background, it essential we study the Bible and see who the true God of Christianity really is. Israel worshipped the gods of the nations over and over again, so we should not be surprised to find that the churches have done the same thing.
In harmony with other religions, Rome adopted the trinity, with an emphasis on the Madonna and Child.
Although the Seventh-day Adventist teaching of the Trinity is not exactly the same as the Roman Catholic Trinity, the use of the symbol is used in a Pictorial Aid for Bible Study.
My Catholic Faith by Louis LaRavoire Morrow p30
However, the counterfeit began many years earlier in Babel, when the worship of the mother and her child became an act of religion. This was the work of Nimrod and his wife Semiramis. The religion was more fully established after the death of Nimrod, when Semiramis became pregnant, supposedly from the sun. Her child was said to be the rebirth of Nimrod in another form as the son of the sun. He was called Tammuz. Thus there were always three in this false religion -- the father, the mother, and the son, with an emphasis on the mother and her child.
In Japan, the Buddhists worship their great divinity, Buddha, with three heads, in the very same form, under the name of “San Pao Fuh” Two Babylons p17.18.
I do hope you will take it seriously because it relates to who we worship. It isn't just a subject to debate, it is salvational.
Taken from the above internet book
Christianity uses the triquietra because the doctrine of the trinity was accepted by the church during the early centuries when many pagans were converting to Catholic Christianity.
In the First Council of Nicaea (AD325) it was formally accepted that the Son was the same substance as the Father. A number of years later (AD381), the Second Council of Constantinople that the Holy Spirit was also of the same substance, making the Father, Son and Holy Spirit the one God, and inserted it into the Nicene Creed. Thus the Trinity became officially the doctrine of Rome.
The Encyclopedia Britannica states, "Another deity, Brahma, the creator, remains in the background as a demiurge. These three great figures (Brahma, Vishnu, and Shiva) constitute the so-called Hindu Trinity -- Trimurti, ‘The One or Whole with Three Forms’ ” Encyclopedia Britannica. Article, Hinduism.
Another symbol is used on the cover of a Seventh-day Adventist book entitled 'The Trinity' by Woodrow Whidden, Jerry Moon and John Reeve - three authors! '
Pictorial Aid by Frank Breaden p74
Why would Christians use a pagan symbol to represent the God of heaven?
Other related triquetra symbols are below.
In India, the supreme divinity…. is represented with three heads on one body, under the name of “Eko Deva Trimurtti -- One God, three forms”.
“From Babylon, this worship of the Mother and the Child spread to the ends of the earth. In Egypt, the Mother and the Child were worshipped under the names of Isis and Osiris. In India, as Ini and Iswara; in Asia, as Cybele and Deoius; in Pagan Rome, as Fortuna and Jupiter, the boy; in Greece, as Ceres, the Great Mother, with the babe at her breast, or as Irene, the goddess of Peace, with the boy Plutus in her arms; and even in Tibet, in China, and Japan, the Jesuit missionaries were astonished to find the counterpart of Madonna and her child…” Two Babylons by Alexander Hyslop p20.21. The research done by Hislop is amazing. Another man did research on his research and found it to be accurate. It is not a book to read as a story, but for research. An excellent source book for the diligent student on the gods of the nations. It is available on the internet.
I have a dear friend who was a Catholic. As a child she could not understand how God could have three heads and we have only one. After all, we were created in His image! What a relief, at the age of 70, when she learned about the true God and His Son.
Roman Catholics use the symbol more than any other denomination, but all may have used it at some stage in their history. The Anglican Church uses it on Trinity Sunday at least once in the year. Below are two Catholic drawings for students.
Edward Gibbons stated, “If paganism was conquered by Christianity, it is equally true that Christianity was corrupted by paganism. The pure Deism of the first Christians…. was changed, by the church of Rome, into the incomprehensible dogma of the trinity. Many of the pagan tenets, invented by the Egyptians and idealized by Plato, were retained as being worthy of belief.” History of Christianity, Edward Gibbon. Preface.
The triquetra represents the Trinity to those who believe in the doctrine.
The above is a simple version -- a circle to donate one eternal God, and a triangle to show three Persons within that circle of eternity.
Ancient Egyptian trinity -- Horus, Osiris, Isis. 2nd millenium BC