10 Listen then to the Unbegotten Father, listen to the Only-begotten Son. Hear His words, The Father is greater than I, and I and the Father are One, and He that has seen Me has seen the Father also, and The Father is in Me and I in the Father, and I went out from the Father, and Who is in the bosom of the Father, and Whatsoever the Father has He has delivered to the Son, and The Son has life in Himself, even as the Father has in Himself. Hear in these words the Son, the Image, the Wisdom, the Power, the Glory of God.
Next mark the Holy Ghost proclaiming Who shall declare His generation? Note the Lord's assurance, No one knows the Son save the Father, neither does any know the Father save the Son and He to whom the Son wills to reveal Him. Penetrate into the mystery, plunge into the darkness which shrouds that birth, where you will be alone with God the Unbegotten and God the Only-begotten. Make your start, continue, persevere. I know that you will not reach the goal, but I shall rejoice at your progress. For He who devoutly treads an endless road, though he reach no conclusion, will profit by his exertions. Reason will fail for want of words, but when it comes to a stand it will be the better for the effort made.
Source: On the Trinity (New Advent)