The imputation of Christ's righteousness is typically defended in light of arguments for the penal substitutionary theory of the atonement and/or the doctrine of salvation by faith alone. In this context, perhaps the three strongest and most commonly cited passages used to extend to this particular aspect of justification are Romans 5 (particularly verse 19), 2 Corinthians 5:21, and Philippians 3:8–9. We'll look at each of those in some detail, and then following that briefly mention a few other relevant verses.

Romans 5

John Murray, in Redemption: Accomplished and Applied, argues that justification of believers entails two things – first, God makes or constitutes Christians righteous, and then he declares them righteous. The imputation of Christ's righteousness is a key aspect of the first of these actions, and Romans 5, particularly verse 19, is seen as evidence for this:

For as by the one man's disobedience the many were made sinners, so by the one man's obedience the many will be made righteous. (ESV)

That is, the obedience of the "one man," Jesus, makes (or constitutes) many people righteous. Murray continues:

In Romans 5:17 [Paul] speaks of those who receive "the free gift of righteousness" and in verse 18 of the judgment which passes upon men unto justification of life "through one righteousness." It is clear that the justification which is unto eternal life Paul regards as consisting in our being constituted righteous, in our receiving righteousness as a free gift, and this righteousness is none other than the righteousness of the one man Jesus Christ; it is the righteousness of his obedience.

Romans 5 is particularly relevant, Charles Hodge says, because in it Paul draws a parallel between Adam's sin ("the one man's disobedience") with Christ's righteousness. Thus, he says, the scriptural basis for the imputation of Adam's sin to mankind can also be employed to defend the imputation of Christ's righteousness.

2 Corinthians 5:21

Another commonly cited passage is 2 Corinthians 5:21:

For our sake he made him to be sin who knew no sin, so that in him we might become the righteousness of God. (ESV)

Like Romans 5:19, some might argue that "righteousness of God" in this verse refers to increasingly moral behavior in the believer. But Hodge argues that such an interpretation causes the analogy to break down:

As Christ was not made sin in a moral sense; so we are not (in justification) made righteousness in a moral sense. As He was made sin in that He “bare our sins;” so we are made righteousness in that we bear his righteousness. Our sins were the judicial ground of his humiliation under the law and of all his sufferings; so his righteousness is the judicial ground of our justification. In other words, as our sins were imputed to Him; so his righteousness is imputed to us. If imputation of sin did not render Him morally corrupt; the imputation of righteousness does not make us holy or morally good.

Philippians 3:8–9

For [Christ's] sake I have suffered the loss of all things and count them as rubbish, in order that I may gain Christ ⁹ and be found in him, not having a righteousness of my own that comes from the law, but that which comes through faith in Christ, the righteousness from God that depends on faith (ESV)

Wayne Grudem and Hodge both point to this passage as well, and Hodge explains why it should be interpreted as teaching the imputation of Christ's righteousness:

[Paul] “suffered the loss of all things,” that he might be found in Christ, not having his “own righteousness, which is of the law, but that which is through the faith of Christ, the righteousness which is of God by faith.” Here again one’s own righteousness is contrasted with that which is of God. The word must have the same sense in both members. What Paul trusted to, was not his own righteousness, not his own subjective goodness, but a righteousness provided for him and received by faith.

Other passages

Charles Hodge and others mention 1 Corinthians 1:30, saying that it indicates a distinction between receipt of righteousness via imputation and the process of sanctification. John Murray cites Isaiah 61:10 ("he hath clothed me with the garments of salvation, he hath covered me with the robe of righteousness") and Isaiah 54:17 ("their righteousness is of me, saith the Lord") as evidence for God being the source of the Christian's righteousness. And Wayne Grudem argues that Christ's desire to "fulfill all righteousness" in Matthew 3:15 is related as well:

[Jesus] had to "fulfill all righteousness" for our sake; that is, for the sake of the people whom he was representing as their head. Unless he had done this for us, we would have no record of obedience by which we would merit God's favor and merit eternal life with him. Moreover, if Jesus had needed only sinlessness and not also a life of perfect obedience, he could have died for us when he was a young child rather than when he was thirty-three years old.

And in the debates over this doctrine in the Westminster assembly, J. V. Fesko relates how proponents also pointed to passages connecting life with obedience to the commandments, such as Leviticus 18:5 and Matthew 19:17 ("If you would enter life, keep the commandments," ESV). Without the imputation of Christ's obedience of the commandments to them, the argument goes, Christians could not enter life.

Summary

Proponents of the imputation of the Christ's righteousness typically build on their arguments for penal substitutionary atonement and salvation by faith alone and contend that several passages, particularly Romans 5:19, 2 Corinthians 5:21, and Philippians 3:8–9, support the doctrine. To them, the discussion of the righteousness of God/Jesus and its application to Christians in these verses refers to the imputation of Christ's righteousness to believers as a basis for and an aspect of their justification.

References

• Fesko, J. V., Theology of the Westminster Standards, Chapter 7
• Grudem, Systematic Theology, 570–71.
• Hodge, Systematic Theology
• Murray, Redemption: Accomplished and Applied, Chapter 5

See also Robert L. Reymond, Systematic Theology, Chapter 19.