The most important thing to remember when discussing mortal and venial sin is that we must avoid the misunderstanding that mortal sins are simply “worse” sins then venial sins. It is true that some sins are graver than others, but tis is not what makes a sin mortal. There is an
extremely common misunderstanding among Protestants (and many Catholics

) that Catholic theology says that minor things like stealing a quarter aren’t worthy of hell in God’s eyes, whereas major sins like murder are. This is not true.
What is true is that all sins are worthy of hell in and of themselves. However, when Christ’s sacrifice on the cross is applied to us, these sins are forgiven, and we will not be condemned to hell for them. Protestants believe this as well. The difference is that Protestants say that this is all there is to the idea of sin and forgiveness. Catholics recognize that there is more.
You see, our everyday sins are all forgiven by the simple fact that we have Christ’s sacrifice applied to us. Catholics say that this happens through baptism, whereas Protestants (tend to) say that it happens through accepting Christ. This is another arguement. The point is that so long as Christ is in our lives, however He gets there, His sacrifice earns us the forgiveness of all of our sins.
However, this is when we are accepting His forgiveness. We can reject it. If we know that something is sinful and completely against the will of God, but we do it anyways, then that sin becomes more than it otherwise would be; it becomes a rejection of God’s forgiveness. We call this a mortal sin. It is not that the sin is in and of itself
different, but it is that our we turn our hearts completely away from God by the sin. The mortality of the sin is not in the sin itself, but is in our hearts. If we know that something is truly against God but do it anyways, we are making an active decision that committing that sin is more important to us than God. So, committing that sin is a rejection of God.
Your OSAS friend may say that the paragraph above is simply a bunch of Catholic nonsense, but he would be mistaken. It is direct from the Bible. Hebrews 10:26-27 tell us the same thing as I wrote above:
For if we go on sinning deliberately after receiving the knowledge of the truth, there no longer remains a sacrifice for sins, but a fearful expectation of judgment, and a fury of fire that will consume the adversaries.
As you can see, it is in the
deliberate nature of the sin, and in that we have
knowledge of the truth that causes us to lose the benefits of Christ’s sacrifice. Oddly enough, the Catholic Church teaches that three things are required for a sin to be mortal: grave matter,
full knowledge, and
full will(deliberateness). The grave matter is not in the passage, but it can be shown from other Biblical passages. However, just being able to show the will and knowledge is something that I think most Protestants will be extremely surprised at.