Imputation is the theological act by which God legally credits righteousness to a believer based solely on the finished work of Christ, or in the case of the Fall, accounts the guilt of Adam to all mankind. Reformed theology maintains that while Adam's sin is imputed to all, the perfect righteousness of Christ is imputed to the elect by faith alone.
In Reformed theology, the fall of Adam is understood through the doctrine of federal representation. Because Adam served as the root and representative head of all humanity, the guilt of his initial transgression is legally reckoned or WCF Ch. 6 §3 imputed to all his natural descendants. This explains why we are born with a corrupted nature and stand under the consequence of sin, not because we personally committed Adam's act, but because we were in him as our covenant head.
Justification is the gracious reversal of our standing before God. It is not based on any internal merit or good works performed by the believer; rather, it is the act where God pardons our sins and accepts our persons as righteous. This occurs solely because the perfect obedience and satisfaction of Christ are credited to our account by God and received by faith alone, as defined in WSC Q.33 and WLC Q.70.
This exchange is the very heart of the gospel. As Paul writes in 2 Corinthians 5:21, God made Him who knew no sin to be sin for us, that we might become the righteousness of God in Him. We are indeed more sinful than we dared believe, yet through this divine transfer, we are more loved and accepted in Christ than we dared hope. Our obedience is not the cause of our acceptance but the joyful fruit of being already declared righteous in Christ.