You asked this question: "Doesn't scripture say something about that being like crucifying Christ all over again?"
For it is impossible for those who are once enlightened and have tasted the heavenly gift, and have become partakers of the Holy Spirit and have tasted the good word of God and the powers of the age to come if they fall away to renew them again to repentance, since they crucify again for themselves the Son of God, and put him to an open shame.
I assume this is the scripture you are referring to. Please correct me if I'm wrong. Hebrews is dealing with Jews who had accepted Jesus as the Son of God and lived the Christian life, only to return to synagogue where they were required to deny that Jesus is the Son of God, putting Him to public shame. That denial required that they subscribe to the apologetics used by the Sanhedrin to explain away the deity of Jesus, making him a common man and liar. It was more than just a denial of knowing Christ, like Peter, and more than a choice to return to carnal desires because of our sinful nature and temptation. It is even more than what Judas did. The writer of Hebrews was addressing apostasy that literally undid the belief the apostates had had in Christ, and doing it in a very public forum that would influence many people. So yes, I will concede that there is a point of apostasy that people who once believed Jesus is the Son of God and who abided in his commandments can succumb to that will indeed assure damnation. Whether or not you accept my analysis here, it bears pointing out that this passage makes it quite clear that it is possible to be in Christ and lose that relationship through apostasy. It isn't a passage that supports the once saved, always saved position, quite the opposite. This passage also lays to rest the "once saved, always saved" contention that someone who turns away from Christ was never really a Christian in the first place.
...Talk about a cruel, psychological mind-bender to place on someone. Am I really saved, or aren't I? If twenty years from now I slip and fall into apostasy, was everything prior to that an illusion? Where exactly is the assurance of salvation? We humans being the game players that we are, I suppose a fallen Baptist minister could come to his senses after twenty years and say, "I wasn't really saved, so now I can get really saved." After that, he might get to wondering if he was really saved the second time. Certainly it isn't the simple assurance John gives when he says:
...1John 3:24 Now he who keeps his commandments abides in him, and He in him, and by this we know that He abides in us, by the Spirit whom He has given us.
1John 3:23 And this is his commandment, that we should believe on the name of His Son Jesus Christ, and love one another, as he gave us commandment.
1John 5:1-2 Whoever believes that Jesus is the Christ is born of God, and everyone who loves the Father also loves the Son. By this we know that we love the children of God when we love God and keep his commandments.
Now to address the accusation that I am suggesting using Jesus Christ as our personal sacrifice like we would do using animal sacrifices over and over again.
Matt: 18:12-14 ...What do you think? If a man has a hundred sheep, and one goes astray, does he not leave the 99 and go to the mountains to seek the one that is straying, and if he should find it, assuredly I say to you, he rejoices more over that sheep than over the 99 that did not go astray. Even so, it is not the will of your Father who is in Heaven that one of these little ones should parish.
My question here is why Jesus chose to use the word "if." That implies that the lost sheep might not be found, and then what? Now if the sheep wasn't really in danger of being lost, it seems foolish for the shepherd to leave the 99 and go after him. After all, sticking to the OSAS reasoning, either that sheep wasn't part of the flock in the first place, or that sheep can't really be lost. Or, if we are to go to the extreme that I have heard some OSAS apologist go to, nothing that sheep can do will make him lost. As we just read in Hebrews, that isn't the case.
James 5:19-20 Brethren, if anyone among you wanders from the truth, and someone turns him back, let him know that he who turns a sinner from the error of his way will save a soul from death, and cover a multitude of sins.
What kind of death do you think James is talking about here? The wandering brethren is going to die at some point, like we all do. James is obviously talking about spiritual death for a brother in Christ who was in the "truth." It doesn't sound to me like the brethren that wander away needn't worry because they are always saved, nor does it sound like James is talking about someone who was mistaken about really being in the "truth," which can only be someone who is in Christ.
Let me include a few more passages to liven up this debate.
Romans 11: 21-23 "Therefore consider the goodness and severity of God: on those who fell, severity; but toward you, goodness, If you continue in His goodness. Otherwise you also will be cut off. And they also, if they do not continue in unbelief, will be grafted in, for God is able to graft them in again."
John 15:6 If anyone does not abide in Me, he is cast out as a branch and is withered: and they gather them, and throw them into the fire, and they are burned. ...Of course, we need to go back to get the full context, where Jesus said, I am the vine. You are the branches. He who abides in me and I in him, bears much fruit.
1John 2:3 Now by this we know that we know Him, if we keep his commandments. vs 4 He who says I know him, and does not keep his commandments is a liar, and the truth is not in him. 2John 9:1 Whoever transgresses and does not abide in the doctrine of Christ does not have God. He who abides in the doctrine of Christ has both the Father and the Son.
I've gone on enough for the moment, so tell me: Do you believe people who fall away from Christ were never truly in Christ in the first place, or do you believe that a Christian can be completely carnal, and still be saved?
Indeed. And then in Jude, verse 24, it says "Now unto him that is able to keep you from falling, and to present you faultless before the presence of his glory with exceeding joy". Hmmm. . .that seems to be saying that it is the Lord who keeps us from falling, not we ourselves.
I will say that Judas appeared to everyone except the Lord to be a believer, but as we know, he wasn't. On the other hand, Peter, who we know was saved, denied Christ three times on the night Jesus was crucified. At an earlier time, Jesus had said to Peter "get thee behind me, satan".
Neither you nor I can know for sure whether someone is truly in Christ or not. We can only judge based on the fruit we see (or think we see), and then we can behave as the bible tells us to.
I'm not exactly sure what you mean by "completely carnal". I've seen all kinds of things called "carnal" that I would not classify in the same way.
I guess it's a good thing to leave the judgment of our eternal souls to the Lord. :)