What does it mean that one of the key adjectives used is ineffable?
In his nature, God is ineffable to human mind. But the Church teaches that, in the "economy of our salvation" God revealed himself as three Hypostases, distinct and unconfused. In his nature, according to the Church teaching, God is a simple, indivisible monad.
In addition, the miracle of Incarnation resulted in the second Hypostasis of the Godhead taking on human nature, human soul and body, and becoming a man, while retaining his fully divine, eternal, uncircumscribed, greater than creation nature.
Furthermore, the Church teaches that the Father is without a cause. He eternally begets his Word and the Spirit eternally "effuses" or sprites from the Father and goes to the Son. As regards their existence, the Word (Son) and the Spirit proceeds form the Father and the Son is begotten of the Father.
However, the Hypostaese are co-equal, co-eternal and con-substantial (of same ineffable essence).
So, while the element of mystery (ineffable) is retained, the Church went into pretty detailed desxcription of the Triune God, none of which, of course is to be found in the Bible, but is derived from the "deposit of faith," believed "everywhere and always."