Great question. First, I wish you all the best in your dialogue.
The good news is that the majority of people are on the same page. Oddly enough, I found your question intriguing so I thought that I would reply. I usually do not, but here goes.
One sentence or less
Done.
Can’t be done. I have run into the same question, and the answer that I have given did not work. I stated an answer that was with in the RCCs belief, but the person being anti-Catholic said “That is not what the Church teaches.”
To me, we are saved by God’s grace alone through Christ Jesus alone in faith. First two are a given, but what about the third? It has to be explained to make our point so that we do not sound like the opposition.
That is why an anti-Catholic will never accept it because of how Catholics and certain Fundy’s understand faith. Diametrically opposed.
In a Catholic sense faith is active in that it holds hope and love (charity) in side of it with the intellectual knowlege that “Christ has died, Christ is risen, Christ will come again.” We admit this every Sunday. As someone in an early post said “faith working in love” (Gal. 5:6) or faith filled works or works of faith (I would be careful of using the latter with anti-Catholics) shows the fullness of what faith does. After all it is an obediance to God. But where the biggest difference is is that Fundy’s see this Catholic concept pact with legalism and wrong interpretation of the “faith” issue. A lot of it is what I like to say is the “nice guy statement” that so many Catholics have stated for so long- “Why are you going to heaven?” the Fundy asks. “Because I am a nice guy. I help little old ladies cross the street, I donate to the Church, I give bla bla bla bla bla.” I used to be one, so I am to blame as well.
Where as to a Fundy, it is faith, faith , faith. Nothing prior to or nothing after makes a difference. All one has to do is have faith (knowlege) of Jesus and accept that and it is done. Eternally secured. I know this well.
However, a Catholic has the knowlege, but is expected to go out and do. Expected because that is what God expects and we should go willingly using his grace. To a Fundy it is looked at as the faith brings about a wanting. The difference being, and I am paraphrasing here, a Fundy, as long as a person has a saving knowlege of Jesus, he/ she does not have to do a darn thing. Now not doing a thing would be looked upon as questionable about his/ her faith, but as long as that someone proclaims the name of Jesus it is always going to take precedence. They may just be seen as lazy. And since all is forgiven, no worries.
Where does all of this leave us. Salvation is not a one sentence answer. To reduce it so diminshes the work of Jesus in my opinion. Please do not take this statement the wrong way. I wanted to warn you away from the easy way out syndrom that is so appealing.
Anyone, let me know if I am off the mark on this.
What a great way to put it. 
