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)

Joel R. Beeke: Faith is instrumental, not the grounds of justifying righteousness, for the saved man believes unto the perfect righteousness of Christ

“Paul confirms in Romans 4 and Galatians 3:6-14 that the imputed (i.e. reckoned) righteousness of Genesis 15:6 is to be understood in terms of ‘by faith or through faith.’ … It is clear then that when Abraham was justified by his faith, the righteousness which was reckoned or ‘charged to his account’ was a righteousness not his own but that of another, namely, the righteousness of Christ (Galatians 3:16). … In these verses the Greek preposition eis does not signify ‘in the stead of,’ but always means ‘with a view to’ or ‘in order to.’ It could be translated ‘towards’ or ‘unto.’ Its meaning is clear from Romans 10:10, ‘with the heart man believeth unto [eis] righteousness’—i.e., faith moves toward and lays hold on Christ Himself.” (Dr. Joel R. Beeke, “The Relation of Faith to Justification,” in the book, “Justification by Faith Alone,” pp. 55-56)

Joel R. Beeke: Justification is extrinsic, and sanctification is intrinsic

Joel R. Beeke explains the important distinction between justification and sanctification:

Justification is extrinsic to the sinner saved; sanctification is intrinsic. Justification declares the sinner righteous and holy in Christ; sanctification makes the sinner righteous and holy as fruit from Christ. Justification removes the guilt of sin, having to do with legal status; sanctification subdues the love and power of sin, having to do with spiritual condition. Justification restores to God’s favor; sanctification restores to His image. Justification is a complete and perfect act, a once-and-for-all act in its essence; sanctification is a progressive but incomplete process, not perfected until death. Justification grants the redeemed the title for heaven and the boldness to enter; sanctification gives them the meetness for heaven and the preparation necessary to enjoy it. Justification gives the right of salvation; sanctification gives the beginning of salvation. By grace the justified are what they are in justification; by grace they work what they work in sanctification. Justification is the criminal pardoned; sanctification, the patient healed. (Dr. Joel R. Beeke, “The Relation of Faith to Justification" in Justification by Faith Alone [Soli Deo Gloria, 1995, pp. 82-83])

Joel R. Beeke: Christ's righteousness, alien to the believer, is the formal cause of justification, and faith is the instrumental cause

Joel R. Beeke explains that faith is the instrumental cause of justification and that Christ's righteousness -- alien to the believer and imputed to him -- is the formal or meritorious cause, the ground upon which we are justified:

Protestant theology, on the other hand [set against Romanism], maintained that faith is the instrumental cause of justification, while the alien righteousness of Christ, external to the believer and imputed to him, is the formal cause, i.e., the ground upon which God can justly justify sinners. ‘For he hath made him to be sin for us, who knew no sin; that we might be made the righteousness of God in him’ (2 Corinthians 5:21; cf. Romans 3:26). It is critical to maintain that this formal cause of justification resides in Christ’s righteousness alone, for all the Scriptures dealing with the fundamentally depraved nature of man make clear that there is no righteousness inherent in the natural man upon which a divine verdict of justification could be based. ‘They are all gone aside, they are together become filthy; there is none that doeth good, no, not one’ (Psalm 14:3). For the Reformers, faith was the conscious, personal immediate reliance of a sinner on Christ alone. (Dr. Joel R. Beeke, Justification by Faith Alone, Soli Deo Gloria (1995), p. 90)