As you've no doubt noticed if you've been reading my blog, I've been reading a lot of books lately that are written for and by Evangelical Protestants. One big difference between their beliefs and Catholic belief is the concept of "once saved, always saved", which, as I understand it, means that at some point in his/her life, a person has to make an affirmative decision to follow Christ, but that, if sincere, once that has been done, future sins are irrelevant to salvation. Before you accept Christ, if over the age of reason, you are condemned to hell; after you accept Him, heaven is yours, no matter what your future sins.
Catholics baptize infants; we believe God's grace is a gift freely given, but a gift that can be rejected. In other words, instead of believing that at some point we have to affirmatively accept Christ, we believe that we are always free to reject Him. That rejection takes the form of what we call "mortal sin". We do not agree with Evangelicals who say that all sins are equal; that, for example, God sees me being impatient with my husband as being the same as cheating on him. In order for a sin to be mortal, it has to be serious, you have to know it is serious and you have to freely choose to do it anyway. That being said, redeption is always possible. Repent of mortal sin and God will forgive you.
Why am I thinking of this now? Well lately I've read a couple of books in which the difference in the way Evangelicals and Catholics see serious sin came up. In one book the main character was separated, but not yet divorced and dating a preacher. She questioned whether she would sin by getting a divorce. Her husband was abusive. The preacher's response was that yes, it would be a sin to get divorce, but that she should go ahead and do it, and then repent so that God could forgive her. After all, God wouldn't want her to be unhappy, would He? If you read the author interview in my post on Forsaken, you'll see that the author says that martyrdom is not necessary for salvation. Now, given that freedom of choice is an integral part of mortal sin, saying "give up your faith, or I'll kill you" does limit your freedom, so may limit whether a sin is mortal, but we are also warned against the sin of presumption--the sin of presuming God will forgive you, so figuring it is ok to sin.