Irish, perhaps I misunderstand you regarding your statement "mortal sin" (a concept unknown to the Eastern Church). I'm confident that the Eastern Church understands well that some sins are deadly, and some are not, as the following passage from 1 John 5:16-17 indicates:
Nonetheless, the venial/mortal distinction isn't made.
This hasn't, however, stopped both EC and EO theologians from writing volume on the topic . . .
As far as Sunday obligation, the best explanation that I've seen was on this forum (I think) a few years ago, that we're not obligated by penalty of sin, but rather in the same way we are obligated to breathe, as we can't live without it.
I had to miss going this morning for the second week in a row due to my tweaked back, and I'm going stark raving mad . . . but I can't sit long without serious lumbar support at the moment, nor could I stand that long . . . [oddly, I could drive
there and back without a problem, but I have power lumbar adjustment . . .] [/quote]
Having a tight back myself, I feel for you.
As I said earlier, the whole Western mindset is about law and keeping the law. Soteriology therefore becomes all about breaking the law and punishments. Of course, the epistemelogical end of such thinking is that you start grading sins as to whether they are "mortal" or "venial" and how bad they are (i.e. how much penance one must DO to pay off the debt).
This is utterly alien to Eastern thinking. I remember the first time a Melkite priest heard my confession. At the end, I kind of stood there, waiting. When he realized that I was waiting to understand what form and how much payment for my law-breaking I had incurred, he said something to the effect of "You are forgiven. Beat it!" With a smile. I have since come to understand more about the difference in Eastern and Western thinking on this issue.
Missing a Divine Liturgy is not a mortal sin because we are not under a law. But if you regularly miss Liturgy, it is more of a thermometer which should tell you, your family, and your priest, that you are spiritually sick and need some strong medicine. In the East, salvation is about healing the soul that has been delivered from death by Christ's death, not worrying about some law that says if I take a certain number of bites of Hagen-Daz I am now in a state of mortal sin.