"And my father and mother strengthened me in this my first impression, saying in my presence, I was intended for some great purpose, which they had always thought from certain marks on my head and breast."
Here we see the origin of Turner's grandiose visions. He's operating based upon the superstition of his parents that he must be important. More than likely his parents are projecting their own fear of insignificance as a result of their slavery, but Turner actually believes them. He takes what they say seriously, constructing his entire identity around this grand sense of purpose.
"My grand mother, who was very religious, and to whom I was much attached — my master, who belonged to the church, and other religious persons who visited the house, and whom I often saw at prayers, noticing the singularity of my manners, I suppose, and my uncommon intelligence for a child, remarked I had too much sense to be raised – and if I was, I would never be of any service to any one – as a slave."
Perhaps this is the real motivation for Turner's delusions of grandeur. He does not believe that he can be successful and survive as a slave, having been prophesied to fail. His real gift is in impressing people, so he chooses to use this skill to side-step responsibility. He's fooling these people.
"The manner in which I learned to read and write, not only had great influence on my own mind, as I acquired it with the most perfect ease, so much so, that I have no recollection whatever of learning the alphabet."
As a result of his early nurturing, Turner is privileged to be taught to read and write, a luxury often denied slaves by law. His education reinforces his belief that he is destined for greatness. Since he's set apart from his peers with special privileges like an education, he is the most qualified to be a leader among them. For his masters, this is motivation enough to ensure Turner is educated in a manner which they choose and supervise, in order to prevent his turning rebellious.
"Having soon discovered to be great, I must appear so, and therefore studiously avoided mixing in society, and wrapped myself in mystery, devoting my time to fasting and prayer."
Turner is not a friendly sort. He's moody and wrapped up in himself at all times. Because of his delusions, he devotes all of his time to the spiritual realm. It's as if he is cleansing and preparing himself for some unknown moment of significance. As a consequence of his solitude, however, he dives further into a delusional mental state. By the time he leads the uprising he has lost all touch with reality. His ego-centric beliefs have been insulated for so long that he has reinforced them internally until they have become his reality.
