Suppose the Father existed alone. For a person to exist alone would be bad. A divine person is a perfectly good person, and that involves being a loving person. A loving person needs someone to love.
The Father will bring into existence another divine person with whom to share his rule of the universe. Following tradition, let us call that other person 'God the Son'.
St Augustine wrote that if the Father wished to cause the Son to exist, and was unable to do it, he would have been weak; if he was able to do it but did not wish to, he would have been ungenerous (that is, because he wished to be the only divine person). Hence the Father would not exist at all unless he caused the Son to exist.
The love of the Father for the Son must include a wish to cooperate with the Son in further total sharing with a third member of the Trinity, whom, following tradition, we may call the Holy Spirit, whom they will love and by whom they will be loved.
In the twelfth century Richard of St Victor made this point and gave a further argument for it. He wrote that anyone who really loves someone will seek the good of that person by finding some third person for him to love and be loved by.
Hence the Trinity must have always existed. The perfect goodness of the Father, the Son, and the Holy Spirit means that they love each other without limit.
'In Taoism, which came later, there also appears a striking resemblance to Christianity. The three Holy figures of Taosim, which correspond to the nature of the the Father, Son, Spirit. They are called the ''Three Pure Ones'' and their characters are described as:
The Jade Purity, 玉清; "The Universally Honoured One of Origin"
(Gen 1:1, John 1:1)
The Supreme Pure One 上清; "The Universally Honoured One of Divinities and Treasures"
(Deuteronomy 33:19, Matthew 13:52)
The Grand Pure One 太清; "The Universally Honoured One of Tao and Virtues"
(John 14:26, Galatians 5:22-23)'