It seems I have been summoned by @Hodos from the ether. I am curious RC - what exactly do you think the “Once Saved Always Saved” belief is? As a beginning point of discussion, would you say that it more closely resembles:
a. Since I am saved, I can sin as much as I want because “once saved always saved”; or
b. Since I - if I am actually saved - was “chosen by God before the creation of the world to be holy and blameless in his sight”, I can’t trump God’s sovereignty and “un-choose” myself.
With regard to “Mortal Sin” - Reformed Protestants completely agree with the concept of “Mortal Sin” in that we believe all sin to be deadly. Said another way, a Reformed Protestant doesn’t believe there are any “Venial” sins.
There are clearly some sins that are far more harmful if committed, e.g. 7 deadly sins, to ourselves and others. However, as far as our relationship with God goes any sin at all makes us “enemies” with God and is therefore lethal.