Justification means that God reckons a believer righteous. Justification is by blood (Rom 5:9). That is, the blood of Jesus is the moral basis upon which God is able to justify a believer. By reason of the blood, God is satisfied that the question of sins is fully settled according to the estimation of the judicial requirements of His throne. Justification is by grace (Rom 3:24) because there is nothing of merit in the believer. He is but guilty, yet, by reason of the blood, God justifies the ungodly (Rom 4:5). It is God that justifies (Rom 8:33); God works entirely from Himself (Rom 1:16). Justification is by faith (Rom 3:28; 5:1). A person whose faith relies on Christ risen and the God who raised Him is justified (Rom 4:24 & 25). Justification by works (James 2:24 & 25) is the practical demonstration of the righteousness the believer already has before God and men.
Justification is not the bringing of a person back to innocence. Hence, 'just-as-if-I'd-never-sinned' is hardly a fitting definition. Justification is not simply pardon, and it involves more than just the forgiveness of sins. These could be described as the 'negative effect' of justification. In its 'positive aspect' the believer is reckoned righteous by God, and he is so by being in Christ (2 Cor 5:21).
It is as being taken out of Adam and set in Christ that the believer is looked on by God as righteous, for God sees the believer as He sees his Head and Representative. 'As He is, so are we' (1 John 4:17). The power of God brings the believer out of the old state in which his sins attached to him, into a new state in which his sins are gone, having been put away in the sacrificial work of Christ, and justified. Once he was under the condemnation of God but now he is justified.