Where does Master Samael write about his incarnation as this personality?
From the lecture The Law of the Pendulum by Samael Aun Weor:
On a certain occasion, when I had the physical body that was known as Thomas á Kempis, I wrote in a book entitled, "The Imitation of Christ," the following phrase:
"Thou art none the holier if thou art praised, nor the viler if thou art reproached. Thou art what thou art; and thou canst not be better than God pronounceth thee to be."