The point here is that “we” don’t define that ... God REVEALS that to us, telling us what we CANNOT KNOW, but can only speculate, if we were totally on our own.
The fact of the matter is that we’re not totally on our own, and what we “have defined” (as you put is) is what “God has defined” for us ... and NOT us.
We couldn’t have defined that with any degree of certainty, as it would be TOTAL SPECULATION on our part. Fortunately “God has revealed it to us” ... which is why He gave us his inerrant and infallible word ... so that we could know the things that he wanted us to know.
HOW can one be the “eternal self-existent being” which is outside of all things created and outside of even the “existence” of the parameter of time??!! I have NO IDEA AT ALL!
BUT ... God has revealed that to us, and he has come down to earth and become a human being to talk directly to us. That is how we know.
Well, certainly all we can know about God is what he chooses to reveal through general and special revelation.