Essential has to do with salvation.
Non-essential does not.

Thus in my opinion, eschatology is non-essential. So a Dispensational Preimillenialist believes in justification by grace through faith without salvific works. An amillenialist believes in justification by grace through faith without salvific works. Both agree on essential doctrine yet disagree on eschatology (and yes, I realize they would disagree on other things as well).

Grace is not common.