I've given this a lot of thought and where I do not doubt the sincerity of my Protestant brothers and sisters convictions, I would say that the "Gospel" according to the Catholic Church is more fully formed than what I've heard proclaimed by many Protestants. It's not just saving sinners, but it's the transforming of sinners into the adopted sons and daughters of the Most High God. At the same time there are Catholics who proclaim their own misunderstandings of the Gospel, such as the "Peace and Justice Gospel", the "God loves you Gospel", or the "Believe in Jesus Gospel" (i.e., Jesus and Me Protestantism). As I mentioned above, the post-Vatican II melee did proper catechesis no favors. What it did do, I believe, was to foster to a great extent, the exodus of a good many Catholics to Protestant communities.
First let me say I wholeheartedly believe that there is truth in Protestantism, just not the whole truth. Things like "we are all sinners", "Jesus died for our sins", and "we must believe in Christ to be saved" are not only true, but are Catholic teaching as well. But they are not the full truth. They lack the "rest of the story" as it were, which can open the door to serious doctrinal misunderstandings. For example, The Doctrine of Justification, a well-worn topic of disagreement on the front line of the Catholic/Protestant debate, Luther considered it to be a legal act of juridical imputation, as in a guilty man being acquitted and declared innocent. While this is partially true, aren't we also, as Scott Hahn is fond of saying, "terminally ill patients"? Are we as Christians only at the mercy of a just judge? No, quite to the contrary actually, we are children of a loving Father, something I am sure that my Protestant brothers and sisters can agree with. So for the Catholic, it's not simply God the judge acquitting the guilty, it's God the Father, bringing his prodigal children home. Yes, Christ paid the price we couldn't pay, but He also heals the terminally ill patients and thereby makes us His adopted sons and daughters. As St. Paul says in Galatians chapter 4:
"And because you are sons, God has sent the Spirit of his Son into our hearts, crying, "Abba! Father!" So through God you are no longer a slave but a son, and if a son then an heir."and again in Romans chapter 8:
"...but you have received the spirit of sonship. When we cry "Abba! Father!"Moving forward, it is also held by Protestants, starting again with Luther that we are justified by faith...alone. Many have told me that "saving faith" will produce good works, and that not only will it, but it must by definition. Now a properly catechized Catholic will tell you that good works are necessary for salvation. Here is where we get to the crux of this post...aren't we all really saying the same thing from own perspectives? (Hopefully my Protestant friends will take me at my word that Catholics don't believe that our own works will save us and that we are talking about works done in the grace of God here -- Phil 2:13) Can you see what I'm saying here? Let me take this a bit further...a Protestant who has a friend who falls away from the faith might tell you that this person never had that "saving faith" and was therefore never saved, and would have to really "accept Jesus into their heart" to be truly saved. Whereas a Catholic will tell you that if you commit a mortal sin you have chosen to turn your back on God, the same way as the Prodigal Son did his father, and you must repent of that sin to return to familial communion.
Pope Benedict XVI spoke to this issue in his General Audience on November 19, 2008. Speaking on Luther's Doctrine of Justification, he said,
"For this reason Luther's phrase: "faith alone" is true, if it is not opposed to faith in charity, in love. Faith is looking at Christ, entrusting oneself to Christ, being united to Christ, conformed to Christ, to his life. And the form, the life of Christ, is love; hence to believe is to conform to Christ and to enter into his love. So it is that in the Letter to the Galatians in which he primarily developed his teaching on justification St Paul speaks of faith that works through love (cf. Gal 5: 14). Paul knows that in the twofold love of God and neighbour the whole of the Law is present and carried out. Thus in communion with Christ, in a faith that creates charity, the entire Law is fulfilled. We become just by entering into communion with Christ who is Love."A thorough read of the Holy Father's General Audience is highly recommended, to avoid seeing contradictions where there are none.
In conclusion, sometimes it may seem that when you, as a Catholic are discussing the faith with a Protestant (and vise versa), that your interlocutor is speaking a totally different language. It makes discussions difficult if you don't think you're talking about the same thing when you really are. So here is what I'd ask before you begin such a discussion. Take a moment, before launching into established rhetoric, to try and understand the framework of their faith tradition. While it is important that we work to resolve our doctrinal differences, we will never do it without understanding on what ground we each stand now. Give them the consideration that they, just like you, are a Christian on a journey, a journey to where all of our hearts can finally rest in the Beatific Vision of the Blessed Trinity.