Have you heard of C.S. Lewis' "mere Christianity"?
Supposedly it is a book about the fundamental doctrines that all Christians - regardless of denomination - susbcribe to.
I have to confess that I have not read the book and that I am not planning to read it as of now.
My reason for this is that I can see the very foundation of Christianity in a single word: love.
This statement should be understood in light ot the greatest commandment which has to be believed in by every single person who claims to be a Christian:
Matthew 22:37 Jesus said to him: Thou shalt love the Lord thy God with thy whole heart, and with thy whole soul, and with thy whole mind. 38 This is the greatest and the first commandment. (Douay-Rheims Bible)
So what should be the primary concern of every single Christian? The total love for God.
And here we will have a question frequently asked by protestants: "Is that necessary for salvation?"
Such questions already reveals the difference in attitude between the Catholic and the protestant. It is a fundamental difference. While the Catholic thinks of totality, the protestant focuses only on the things he deems to be necessary for salvation and therefore rejects certain things like the 7 sacraments.
Where is the problem though? The problem lies in the fact that the question as stated above reveals an objective disorder in attitude. A Christian is commanded to love God above all things in totality. What does this mean though? This means that he desires that which is pleasing to God: whether *that* is indeed necessary for salvation or not should not even be our concern. If the answer to that question though is the basis for the protestant's ideology, then love is not really the reason for his belief. We can conclude that someone who seeks only to follow that which he thinks to be necessary for salvation (since all the doctrines and rules Catholics follow are rather inconvenient and numerous) is then not really focused on the love for God, but rather focused on saving his own butt from punishment, eternal damnation. That leads then to the question whether that particular protestant believes in God out of love or out of selfishness (the avoidance of one's suffering).
The Cath0lic does not have this problem: to him, faith is an all-or-nothing package. It is about totality, loving God with his whole heart, mind and soul.
What of him who relies on faith alone in order to have a rather convenient *assurance* to avoid damnation? What of him who asks "is that necessary for salvation"? What of him who does not really believe out of love for the Lord Jesus Christ?
Since protestants cling on to the false teaching of sola scriptura, I will let Sacred Writ answer:
1 Corinthians 16:22 If any man love not our Lord Jesus Christ, let him be anathema, maranatha.