It’s important to remember that most Protestants (especially Reformed Protestants) have a slightly different understanding of the Atonement and a very different understanding of how it is we are saved and it is all tied together with election (predestination). If you encounter Reformed theology very much this will become very evident. As I said before, R.C. Sproul is very much a Calvinist and one of the stronger proponents of Reformed theology in the world today.
In Reformed theology Christ did not die for all men, his Atonement in no way paid for the sins of all men. Rather, they believe that God the Father poured out all of his divine wrath on Jesus on the Cross (‘cursing him’ if you will) for all of the sins of the elect (past, present and future). And, at the same time, Christ’s righteousness is to be ‘imputed’ to the elect so that when God looks at an ‘elect’ Christian he only sees the righteousness of Christ which has been imputed to him, and not the sins (which have all been imputed to Christ on the Cross). It is impossible to lose this state of justification through any sort of sin committed because all sins have been been forgiven their debt of eternal punishment (therefore, they are all essentially venial). Of course, since this is tied in to their understanding of ‘election’ and ‘predestination’ it would be a straw-man to conclude that one could become apostate and still be saved; while this is theoretically possible to conceive logically, it proves to be impossible in this system because they believe that all of the elect will attain to faith and justification (and the elect only - no one that isn’t elect will even be justified) and all of the elect will persevere in faith to the end because they are predestined - no exceptions.
I would recommend reading this article on Called to Communion:
Called to Communion is a very good Catholic site that is primarily devoted to discussing and countering Reformed theology. It was very instrumental in my coming to the Catholic faith. If you think you may have much contact with Reformed theology I would recommend reading some of their articles and the subsequent discussions.