J.I. Packer: Justification was the Material Principle of the Reformation

J.I. Packer on the controlling principles of the Reformation:

Justification by faith has traditionally, and rightly, been regarded as one of the two basic and controlling principles of Reformation theology. The authority of Scripture was the formal principle of that theology, determining its method and providing its touchstone of truth; justification by faith was its material principle, determining its substance. (J.I. Packer, “Introductory Essay” -- in James Buchanan’s Justification, Banner of Truth, 1997 -- reprinted from the 1867 original, p. vii)