You may recall from previous posts that we've pretty clearly established that Dave explicitly rejects the teaching of Vatican II that Muslims and Catholics worship the same god.
Let's turn to justification.
Here it gets interesting, because Dave agrees with Trent that justification is not by faith alone.
However, when Dave goes to define Christianity, what does he use? A creed - that is to say, Dave makes Christianity (definitionally) about faith alone.
What's logical result: under Dave's definition, some Christians are not justified!
But perhaps this is acceptable for Dave.