Here is an excerpt about the nature of justification, very useful and encouraging to think upon what Christ has done for us and how we could never earn His favor, it is only through the finished work of Christ that we are righteous before God.
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).
~J. I. Packer, Concise Theology: A Guide to Historic Christian Beliefs (Wheaton, IL: Tyndale House, 1993)~
James Boice also had a concise yet accurate definition of it in his commentary on Matthew 5:4
Jesus Christ came to step between the wrath of God against sin on the one hand and all who trust in Jesus Christ on the other. He took the blow of God’s wrath upon himself, paying the full penalty for our sin, and God has placed the full righteousness of the Lord Jesus Christ to our account, so that we are seen as being perfectly acceptable before God in him.
~James Montgomery Boice, The Sermon on the Mount: An Expositional Commentary (Grand Rapids, MI: Baker Books, 2002). Page 29 ~