Mark Hausam, a former Presbyterian who converted to Roman Catholicism, has an excellent post on the problematic nature of the Reformed doctrine of justification and imputed righteousness:
A Question for Protestants about Justification
This is a topic I have address many times on this blog, including:
Response to a Recent Attempt to Defend Imputed Righteousness
A Question for Protestants about Justification
This is a topic I have address many times on this blog, including:
Response to a Recent Attempt to Defend Imputed Righteousness