Justification by faith alone is the act of God's free grace wherein He pardons a sinner's transgressions and declares them righteous in His sight. This righteousness is not earned by any human work or inherent quality, but is found entirely in the perfect obedience and satisfaction of Jesus Christ, which is imputed to the believer and received through faith alone.
Justification is fundamentally a legal declaration by God the Judge. As affirmed in the 1689 LBCF Ch.11 §2, faith is the sole instrument by which we receive and rest upon Christ and His righteousness. We are not justified because of anything we have done or any inherent goodness we possess; rather, as stated in the Westminster Shorter Catechism Q.33, God pardons all our sins and accepts us as righteous only because of the righteousness of Christ imputed to us.
The basis of our standing before God is exclusively the finished work of Jesus. We are all, by nature, sinners and godless, yet we are justified by His grace as a gift. As the Second Helvetic Confession Ch.XV notes, we do not share in this benefit partly through our own efforts; we attribute it wholly to the grace of God in Christ. This truth reminds us that we are more sinful than we ever dared believe, yet more loved in Christ than we ever dared hope, for we are recipients of a righteousness that is not our own. As the Apostle Paul writes, Romans 3:23-24.
While we are justified by faith alone, the faith that justifies is never alone. It is a living, working faith that inevitably produces the fruit of obedience. As the 1689 LBCF Ch.11 §2 clarifies, faith is accompanied by all other saving graces and works by love. This safeguards us against both moralism—thinking our works gain us favor—and license—thinking obedience is optional. Because we are accepted in Christ, we now walk in the good works which God prepared beforehand, as referenced in James 2:14-26.