How can I explain the Catholic teaching on justification to Protestants who have a different understanding?

Look up Romans 6:7. All standard English translations render this verse as some variation on the statement, “He who has died has been freed from sin.” The topic here is one of sanctification, the making the believer holy, or freeing him from sin.

What is significant about Romans 6:7 is that when it says the one who has died has been freed from sin, the word for freed is actually the Greek word for justified. What it literally says is, “He who has died has been justified from sin,” yet the context is so obviously sanctificational that all standard English translations of the Bible rendered “justified from sin” as “freed from sin.” This shows that for Paul there was not a rigid wall between justification and sanctification. The meanings of the two terms overlap in his mind.