I would agree that any worthiness would have to be by and through Christ. According to the Bible it is either proper or improper for us to partake, and Paul tells us to determine this we should do an honest assessment of ourselves (has Christ so worked in us). The Bible gives us ample evidence of what a true Christian "looks like" so we have a perfect template for us to use. Of course if we "pass" and figure it is proper for us to partake the credit and glory for this go directly to God, not to ourselves for anything we have done to make ourselves worthy.
So, when I consider myself presumptively saved it is only a reflection of whether I believe God's Biblical promises, as best as I can understand them, to be true or not. The Bible plainly shows us what a true Christian looks like so if an honest assessment leads me to think I look like that then I say I am presumptively saved, not because of me but because God's word is true. And as Metmom just showed with her verses, the Bible also says that we may know and have confidence about this. Thus, the backing for these presumptions is never ourselves, but our understanding of what God's word says and that it is true.
Here is what we Orthodox pray immediately before receiving communion:
Theological differences aside, it's a beautiful prayer.
That's just the problem, FK, "our understanding of what God's word says". The Church's understanding, we may be sure, is correct; our own personal understanding...not so sure. These matters are as we say "up to God". We shouldn't presume anything and hope for everything through God's mercy. I may have mentioned to you over the years, a collection of spiritual writings, mostly but not exclusively by monks, called the Philokalia. The thoughts of +Peter of Damascus, probably from the 12th century, are the most quoted in that book. He wrote for monks and his spiritual children but his observations have value to others, lay people, even today. In a section called "A Treasury of Divine Knowledge", he wrote: "...presumption alone, without any other sin, is enough to destroy the soul; for he who regards his sins as trivial is allowed to fall into those that are greater..." Worth remembering.
"Theological differences aside, it's a beautiful prayer."
Indeed it is.
How do you separate you from your understanding, FK?