By no means a complete list. There are quotes about the Trinity being a mystery that date back to Athanasius. No conjecture. It's very common for people to just say it's a mystery because it's incomprehensible and doesn't fit into Scripture.
It is commonly said that the Trinity is a mystery. And it certainly is … . But it is not a mystery veiled in darkness in which we can only grope and guess. It is a mystery in which we are given to understand that we will never know all there is of God … . It is not a mystery that keeps us in the dark, but a mystery in which we are taken by the hand and gradually led into the light …
—Eugene Peterson, Christ Plays in Ten Thousand Places (Eerdmans, 2005) p. 306
In the deepest mystery of his being, God is an intimate relationship, a fellowship, a community of love.
—Darrell Johnson, Experiencing the Trinity (Regent College Publishing, 2002), p. 51
The Trinity is the central mystery of the Christian faith and life. It is the source of all other mysteries of Christian faith, the light that enlightens them.
—Catechism of the Catholic Church
More than any other Christian doctrine, the Trinity sets before us the mystery of God and points to the element of mystery in every aspect of our faith.
—Stephen Seamands, Ministry in the Image of God (IVP, 2005), p. 100
Seeing mystery…enables us to understand how it provokes reverence, the only possible attitude to what is supreme and final in our lives. It is not a mystery that leaves us dumb and terrified, but one that leaves us happy, singing, and giving thanks. Mystery is like a cliff; we may not be able to scale it, but we can stand at the foot of it, touch it, praise its beauty. So it is with the mystery of the Trinity.
—Leonardo Boff