“And God said unto Moses, I AM THAT I AM’: and he said, Thus shalt thou say unto the children of Israel, I AM hath sent me unto you.” — Exodus, 3.14
As far as we know, only two individuals during the Elizabethan age used the biblical phrase “I AM THAT I AM” to describe themselves, and they did so within identical contexts: the author of Shakespeare’s sonnets and Edward de Vere, Earl of Oxford.
After composing a letter to his father-in-law William Cecil Lord Burghley on 30 October 1584, de Vere signed off in his own hand. Then he added a postscript bitterly protesting the chief minister’s attempts to use his own servants to spy on him. He set forth the facts and continued (with my emphases):
“But I pray, my Lord, leave that course, for I mean not to be your ward nor your child. I serve her Majesty, and I AM THAT I AM, and by alliance near to your Lordship, but free, and scorn to be offered that injury to think I am so weak of government as to be ruled by servants, or not able to govern myself. If your Lordship take and follow this course, you deceive yourself, and make me take another course than yet I have not thought of. Wherefore these shall be to desire your Lordship, if that I may make account of your friendship, that you will leave that course as hurtful to us both.”
(When Oxford warns, “If your Lordship take and follow this course, you … make me take another course than yet I have not thought of,” it appears he anticipates King Lear’s outburst against his two selfish daughters, “I will do such things – what they are yet I know not; but they shall be the terrors of the earth.” – 2.4.280)
The other personal use of I AM THAT I AM occurs in Sonnet 121, which follows here with my emphases on SPIES as well as I AM THAT I AM. Is it the same mind at work … same protest … same angry, accusing voice?
Sonnet 121
‘Tis better to be vile than vile esteemed,
When not to be receives reproach of being,
And the just pleasure lost, which is so deemed,
Not by our feeling, but by others’ seeing.
For why should others’ false adulterate eyes
Give salutation to my sportive blood?
Or on my frailties why are frailer SPIES,
Which in their wills count bad what I think good?
No, I AM THAT I AM, and they that level
At my abuses reckon up their own.
I may be straight though they themselves be bevel;
By their rank thoughts my deeds must not be shown;
Unless this general evil they maintain:
All men are bad and in their badness reign

Dr. Roger Stritmatter’s Dissertation on Oxford’s Geneva Bible: a Landmark in Oxford-Shakespeare Scholarship
God’s words to Moses “I AM THAT I AM” are in the Geneva Bible, a gilt-edged copy of which de Vere had purchased in 1569/70 from William Seres, stationer; and thanks to the landmark studies of that same copy by Dr. Roger Stritmatter, we can be sure the earl was intimately acquainted with its passages. Both Oxford and “Shakespeare” were biblical experts – one more reason why, in the view here, they were one and the same.
Referring to the likelihood that Oxford’s postscript and Sonnet 121 were written virtually at the same time in response to the same situation, Percy Allen wrote in 1930: “So forcible, individual, and wholly characteristic an expression … is a very strong piece of corroborative evidence.” (The Case for Edward de Vere, 17th Earl of Oxford as “Shakespeare” by Percy Allen, 1930)
This reason is now No. 86 in Chapter 15 (“Fingerprints”) of 100 Reasons Shake-speare was the Earl of Oxford.