Simply put, to justify is to declare righteous, to make one right with God. Justification is God’s declaring those who receive Christ to be righteous, based on Christ’s righteousness being imputed to the accounts of those who receive Christ (2 Corinthians 5:21). Though justification as a principle is found throughout Scripture, the main passage describing justification in relation to believers is Romans 3:21-26: “But now a righteousness from God, apart from law, has been made known, to which the Law and the prophets testify. This righteousness from God comes through faith in Jesus Christ to all who believe. There is no difference, for all have sinned and fall short of the glory of God, and are justified freely by his grace through the redemption that came by Christ Jesus. God presented him as a sacrifice of atonement, through faith in his blood. He did this to demonstrate his justice, because in his forbearance he had left the sins committed beforehand unpunished—he did it to demonstrate his justice at the present time, so as to be just and the one who justifies those who have faith in Jesus.”
We are justified, declared righteous, at the moment of our salvation. Justification does not make us righteous, but rather pronounces us righteous. Our righteousness comes from placing our faith in the finished work of Jesus Christ. His sacrifice covers our sin, allowing God to see us as perfect and unblemished. Because as believers we are in Christ, God sees Christ's own righteousness when He looks at us. This meets God's demands for perfection; thus, He declares us righteous—He justifies us.
Romans 5:18-19 sums it up well: “Consequently, just as the result of one trespass was condemnation for all men, so also the result of one act of righteousness was justification that brings life for all men. For just as through the disobedience of the one man the many were made sinners, so also through the obedience of the one man the many will be made righteous.” It is because of justification that the peace of God can rule in our lives. It is because of justification that believers can have assurance of salvation. It is the fact of justification that enables God to begin the process of sanctification—the process by which God makes us in reality what we already are positionally. “Therefore, since we have been justified through faith, we have peace with God through our Lord Jesus Christ” (Romans 5:1).