Perfection of the Law
The law could not be supplemented or corrected without convicting it of imperfection and faultiness (and so charging that faultiness upon God, the author of the law). Even the thought of this is impious.
Francis Turretin, Institutes of Elenctic Theology, ed. James T. Dennison Jr., trans. George Musgrave Giger, vol. 2 (Phillipsburg, NJ: P&R Publishing, 1992-1997), 20.