Once Saved, Always Saved?

Someone recently pointed out that they didn't agree with a given church's stance on "once saved, always saved," the thought that once one has accepted Jesus Christ, one is forever "with Him," regardless of future actions.

They were pointing out, as an example, Hebrews 6:4-8, where Paul writes:

It is impossible for those who have once been enlightened, who have tasted the heavenly gift, who have shared in the Holy Spirit, who have tasted the goodness of the word of God and the powers of the coming age and who have fallen away, to be brought back to repentance. To their loss they are crucifying the Son of God all over again and subjecting him to public disgrace. Land that drinks in the rain often falling on it and that produces a crop useful to those for whom it is farmed receives the blessing of God. But land that produces thorns and thistles is worthless and is in danger of being cursed. In the end it will be burned. (NIV)

The key phrases here are "It is impossible for those who've tasted the goodness of the word of God and who have fallen away, to be brought back to repentance."

That's a heck of a statement. Does it mean that one who accepts Christ, denies Christ, and then repents is lost?

I don't think so; Peter denied Christ and certainly was numbered among the saints.

So what does "falling away" mean?

I don't know. The Bible speaks of an unforgivable sin, but doesn't really clarify exactly what it is. (One is "blaspheming the Holy Spirit," but even there, there's room; what is blaspheming? Plenty of Christians come to Christ after saying there is no God, or that God is ridiculous, or claiming other gods.)

Here's how I see it.

We are all sinners. ("For all have sinned and fallen short of the glory of God.")

Through God's grace, He accepted a substitutionary sacrifice for propitiation of our sins, such that we could be redeemed. At one period, this was through sacrifice of animals, using their blood to cover the sins of Man.

Jesus was born as the Son of God (and God-made-man), and died for us as the perfect atonement for all who accept His death as propitiation for their sins. With this sacrifice of a perfect innocent whose blood covers all, the animal sacrifice was abolished and salvation comes through Jesus and Jesus alone.

To me, there's no sin such that Jesus' death was not enough to cover it. Jesus never said "Sorry, you can't join Me in Heaven" such that the one spoken to literally had no choice. (It was more like "If you follow Me, you will be with Me; if not, well...")

If you think of salvation as a series of states (a finite state machine), you have a graph that looks like this:

I don't see how Man - if he's unable to save himself - can sin such that there's a loop in this graph. We don't control our salvation at its start - God saves us, we don't save ourselves. If we can damn ourselves, then we are able to have more power than the Bible says we should have.

I understand those who feel that there's a way to lose your salvation - but to me, if that's possible, then we lose our salvation immediately after gaining it, through our sin and failure. (We're covered by the blood, redeemed, not perfect in and of ourselves.)