Packer and Beeke: Faith is not the ground of justification, but rather the instrument of receiving and resting in the person and work of Christ alone

“When Paul paraphrases this verse [Gen. 15:6] as teaching that Abraham’s faith was reckoned for righteousness (Rom. 4:5, 9, 22), all he intends us to understand is that faith—decisive, whole-hearted reliance on God’s gracious promise (vss. 18ff)—was the occasion and means of righteousness being imputed to him. There is no suggestion here that faith is the ground of justification.”
-J.I. Packer (quoted by Dr. Joel R. Beeke in “Justification by Faith Alone,” pp. 56-57)

“… Habakkuk 2:4 … quoted in Romans 1:17, Galatians 3:11, and Hebrews 10:38, is ultimately fulfilled in the righteousness that comes by faith in the gospel of Christ, for which the law itself teaches us to look (Romans 3:21-22; 10:4). Paul’s explanation of Habakkuk has inspired not only Martin Luther but countless other believers to place their faith in a righteousness not their own, but that of Jesus Christ who is called ‘THE LORD OUR RIGHTEOUSNESS’ (Jeremiah 23:6).”
-Dr. Joel R. Beeke, “The Relation of Faith to Justification” (in the book, “Justification by Faith Alone,” pp. 57-58)

J.I. Packer: The penal substitutionary view of the atonement is the foundation of Christ's righteousness imputed to the believer in justification

"Salvation in the Bible is by substitution and exchange: the imputing of men’s sins to Christ, and the imputing of Christ’s righteousness to sinners. By this means, the law, and the God whose law it is, are satisfied, and the guilty are justly declare immune from punishment. Justice is done, and mercy is made triumphant in the doing of it. The imputing of righteousness to sinners in justification, and the imputing of their sins to Christ on Calvary, thus belong together; and if, in the manner of so much modern Protestantism, the penal interpretation of the Cross is rejected, then there is no ground on which the imputing of righteousness can rest." (J.I. Packer, Introductory Essay in James Buchanan’s “Justification,” published by Banner of Truth, p. xiii)

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)