I think this is particularly troublesome for Protestants, who generally seem to relegate salvation to a status, or declaration of "not guilty" before the courts of Heaven, the main proof of which is "correct belief," as opposed to something that genuinely involves the whole person, and is an actual ontological reality.
The former takes a line of reasoning like this: "Boy, I sure hope when death comes knocking that I'm stilling mentally agreeing with the factual truths of the Christian faith, namely that as long as I believe in Jesus (whatever that happens to mean) that I have been declared not guilty and thus have no reason to fear the penalty of a guilty status." It's almost as if it's a race against time, where the goal is to "believe" until you die, before the buzzer goes off on your life, and God unscrews your head and looks deep into your brain to make sure you really
believed Jesus to be the Way, Truth and Life.
The latter, however, (what one might call the Orthodox understanding, or the truly Apostolic understanding) is a synergistic relationship between the human creature and God, wherein we physically and spiritually participate in God's energies (grace), through the Sacraments (primarily the Eucharist), actually reaping the tangible results of that saving experience. And in this inter-relationship, we are both physically and spiritually redeemed from death, to the point where our Saint's actual bodies often times do not experience corruption, and smell of myrrh. Something is genuinely happening, but it cannot happen unless we enter into that participatory reality.
So, though we can never really be "certain" or "sure" of where we'll end up (because there's always the chance that we'll turn away from God, in the end), those who "partake of the divine nature," which produces tangible results in that we are freed from "the passions" and take on the character and likeness of Christ Himself, in our entirety, body, soul and spirit, are the ones who have the promise of eternal life.
I typed this out really fast, as I'm short on time, and I may have mis-worded some things, so let me know if this doesn't make sense, or sounds incorrect, etc. etc.