This article by Dr. J. I. Packer, found in “Concise Theology: A Guide to Historic Christian Beliefs (Carol Stream: Tyndale House Publishers, 1993, pp. 164-66) is one of the most concise and accurate descriptions of justification I have come across. His explanation of the Roman Catholic view and how it differs from the Protestant view is also very useful.

“The doctrine of justification, the storm center of the Reformation, was a major concern of the apostle Paul. For him it was the heart of the gospel (Rom. 1:17; 3:21-5:21; Gal. 2:15-5:1) shaping both his message (Acts 13:38-39) and his devotion and spiritual life (2 Cor. 5:13-21; Phil. 3:4-14). Though other New Testament writers affirm the same doctrine in substance, the terms in which Protestants have affirmed and defended it for almost five centuries are drawn primarily from Paul.

Justification is a judicial act of God pardoning sinners (wicked and ungodly persons, Rom. 4:5; 3:9-24), accepting them as just, and so putting permanently right their previously estranged relationship with himself. This justifying sentence is God’s gift of righteousness (Rom. 5:15-17), his bestowal of a status of acceptance for Jesus’ sake (2 Cor. 5:21).

God’s justifying judgment seems strange, for pronouncing sinners righteous may appear to be precisely the unjust action on the judge’s part that God’s own law forbade (Deut. 25:1; Prov. 17:15). Yet it is in fact a just judgment, for its basis is the righteousness of Jesus Christ who as ‘the last Adam’ (1 Cor. 15:45), our representative head acting on our behalf, obeyed the law that bound us and endured the retribution for lawlessness that was our due and so (to use a medieval technical term) ‘merited’ our justification. So we are justified justly, on the basis of justice done (Rom. 3:25-26) and Christ’s righteousness reckoned to our account (Rom. 5:18-19).

God’s justifying decision is the judgment of the Last Day, declaring where we shall spend eternity, brought forward into the present and pronounced here and now. It is the last judgment that will ever be passed on our destiny; God will never go back on it, however much Satan may appeal against God’s verdict (Zech. 3:1; Rev. 12:10; Rom. 8:33-34). To be justified is to be eternally secure (Rom. 5:1-5; 8:30).

The necessary means, or instrumental cause, of justification is personal faith in Jesus Christ as crucified Savior and risen Lord (Rom. 4:23-25; 10:8-13). This is because the meritorious ground of our justification is entirely in Christ. As we give ourselves in faith to Jesus, Jesus gives us his gift of righteousness, so that in the very act of ‘closing with Christ,’ as older Reformed teachers put it, we receive divine pardon and acceptance which we could not otherwise have (Gal. 2:15-16; 3:24).

Official Roman Catholic theology includes sanctification in the definition of justification, which it sees as a process rather than a single decisive event, and affirms that while faith contributes to our acceptance with God, our works of satisfaction and merit contribute too. Rome sees baptism, viewed as a channel of sanctifying grace, as the primary instrumental cause of justification, and the sacrament of penance, whereby congruous merit is achieved through works of sanctification, as the supplementary restorative cause whenever the grace of God’s initial acceptance is lost through mortal sin. Congruous, as distinct from condign, merit means merit that it is fitting, though not absolutely necessary, for God to reward by a fresh flow of sanctifying grace. On the Roman Catholic view, therefore, believers save themselves with the help of the grace that flows from Christ through the church’s sacramental system, and in this life no sense of confidence in God’s grace can ordinarily be had. Such teaching is a far cry from that of Paul.”