This comes from the book New Seeds of Contemplation by Thomas Merton, a Trappist monk.
The passage continues:
“If, therefore, I do anything or think anything or say anything or know anything that is not purely for the love of God, it cannot give me peace, or rest, or fulfillment, or joy.
To find love I must enter into the sanctuary where it is hidden, which is the mystery of God. And to enter into His sanctity I must become holy as He is holy, perfect as He is perfect.
How can I even dare to entertain such a thought? Is it not madness? It is certainly madness if I think I know what the holiness and perfection of God really are in themselves and if I think that there is some way in which I can apply myself to imitating them. I must begin, then, by realizing that the holiness of God is something that is to me, and to all men, utterly mysterious, inscrutable, beyond the highest notion of any kind of perfection, beyond any relevant human statement whatever.
If I am to be “holy” I must therefore be something that I do not understand, something mysterious and hidden, something apparently self-contradictory; for God, in Christ, ’emptied Himself.'”—Thomas Merton, New Seeds of Contemplation