Why does Christianity need a mystery when the Bible is quite clear one is unnecesary?

Some random quotes from online Trinitarian sources. >The mystery of the Most Holy Trinity is the central mystery of the Christian faith and life. >The most profound and mysterious doctrines of the Christian faith: the Trinity. >The mystery of the Trinity doctrine is the manifestation of three distinct persons in one God. Given the very specific statements of Jesus who declared the Father was the only true God, with no mention of a spirit 'person', and literally *excluding* himself from being God, this biblical evidence from the 'only Savior and Lord' would seem a sufficient explanation. The Biblical evidence describing the one God, Yahweh, is not a mystery, Jesus is not a mystery especially as his God is the same as ours. The Apostles affirmed these simple truths, especially about the man Jesus, who again, is never described as God, so no mystery there either.