The Definitive Case Against Definitive Justification?

In his reflections on his White Horse Inn interview conducted by Michael Horton, Catholic apologist Robert Sungenis complains that he offered up a perfect test case for the Catholic doctrine of justification, and Horton kept ignoring it. He says:

On the matter of David’s justification that St. Paul mentions in Romans 4:5-8, I mentioned the fact (as I do in my book) that it is obvious from Paul’s treatment of the account that David lost his justification by committing adultery and murder (2 Samuel 11-12) and that these sins are the very reason that Paul can then use David as an example of a person who, after he committed these heinous mortal sins, can now receive justification when he repents of those sins. If there is any passage of Scripture that supports the Catholic understanding of Justification, this is it. But to my dismay, Horton made no comment about the obvious conundrum the account of David’s justification creates for his own “one-time forensic event” and “eternal security” beliefs to which he holds so dearly in his Reformed theology, even though I referred to David’s account THREE times in the interview. Rather, each time I mentioned David he quickly turned to another topic of discussion and pretended as if the example of David didn’t even make a dent in his view.

As I understand it, here is Sungenis’s argument in a nutshell:

Major Premise: Paul cites David as an example of his doctrine of justification, quoting from Psalm 32 (“Blessed is the man whose lawless deeds are forgiven, etc.”).

Minor Premise: The event to which David is referring is his adultery with Bathsheba and murder of her husband Uriah.

Conclusion: Since David was described as “a man after God’s own heart” prior to this sinful episode, Paul’s calling David’s receiving of forgiveness “justification” demonstrates that, for the apostle, justification Is not a one-time event that occurs at the beginning of the Christian life, but rather is a process that is repeated with each new occurrence of mortal sin and receiving of forgiveness.

What do you think of Sungenis’s case? Is it strong? Weak? And why?