Actually, I don't think there's any way you can start a sentence with "I am thee", unless you are a conservative Quaker.
"Thee" is the objective case, while "thou" is subjective. For a transitive verb, you'd want the objective case ("I love thee, God"). But "to be" isn't a transitive verb, so you need the subjective case ("I am thou, God").
The exception -- and being the persnickety bastard that I am, I positively writhe with delight at being able to point this out -- is that, among the Religious Society of Friends, while "thee" and "thy" were preserved as part of what was called "plain speech", "thou" gradually disappeared as "thee" became standard usage for both the subjective and objective cases.
So "I AM THEE, GOD" would be acceptable usage among traditionalist Quakers, if there are still any left who use plain speech. But for the rest of us, it should be "I AM THY GOD" or "I AM THOU, GOD".
 See: Maxfield, E.K. 1929. Quaker "Thou" and "Thee". American Speech, 4(5): 359-361.