Taken Entirely from Monergism Blog: Article entitled
Resting in the Righteousness of Christ
To be united to Christ by grace though faith is to be counted as having lived His life, obeyed His law, suffered His punishment, died His death, and risen in His resurrection. The believer is not merely inspired by Christ; he is represented by Him. Christ’s perfect obedience was rendered on our behalf; He fulfilled every command of God’s law in our place (Romans 5:19; Galatians 4:4-5). His death was not merely an act of martyrdom. It was the full exhaustion of the law’s curse for our sin (Galatians 3:13). And His resurrection from the dead is the public declaration from God that the believer is now justified, fully accepted and acquitted in His sight (Romans 4:25; 1 Timothy 3:16).
This means justification is not a process, nor is it contingent upon our progress. It is a once-for-all verdict, grounded in the finished work of Christ, granted before we perform any act of obedience or experience any inward renewal (Titus 3:5-7). Our status before God does not rise and fall with our spiritual condition. It is as unchanging as the righteousness of Jesus Himself (Hebrews 13:8; 1 Corinthians 1:30). The gospel, then, assures trembling sinners that the moment they are in Christ, they are as justified as they ever will be (Romans 8:1; Colossians 2:10).
Such a gospel dismantles every attempt to mingle Christ’s righteousness with our own works. Even the slightest reliance on our obedience (whether in gaining or maintaining favor with God) undermines the very grace of the gospel (Galatians 2:21). It dishonors Christ by implying that His obedience was insufficient (Hebrews 10:14). Worse still, it leaves the conscience without peace, for if our standing with God is even partially based on our performance, we will never rest, never know security, and never truly be free from guilt (Romans 5:1; Hebrews 9:14).
But faith ... true, saving faith ... looks away from self entirely. It does not contribute; it receives. It rests wholly on the righteousness of Another (Philippians 3:9). This righteousness is not infused, earned, or grown into. It is imputed, credited by God Himself (Romans 4:5-6; 2 Corinthians 5:21). It is alien to us, but ours by grace. In trusting Christ alone, the believer finds unshakable peace, because his salvation rests not on shifting sands, but on the finished and faultless work of the Son of God (Isaiah 32:17; John 19:30).