Yeah, you should laugh. It's a ridiculous concept.
Hebrews 6:4-6 is absolutely about salvation, but it's written in a (not atypically) hyperbolic way. The context is (Jewish) Christian believers who have been fully initiated returning to (non- and anti-Christian) Judaism.
(Note that the historical context of Hebrews is almost certainly the Roman invasion of AD 70. Jewish Christian believers are under intense pressure to return to Judaism and "put on the uniform" to fight the Romans.)
The hyperbolic part is the assertion that is is "impossible" to bring such people to repentance. Of course it's not literally "impossible". (cf "nothing is impossible for God") The non-hyperbolic statement would closer to "unlikely to work; don't waste your time".
And yes, such people absolutely "lose" (I detest that word; they threw it away quite deliberately) their salvation, unless you want to content that they were "never truly saved" in the first place. But how would you -- objectively speaking -- ever know that except after-the-fact?
In fact, the Temple is referred to in the present tense several times, But no mention of Jerusalem under siege, which started in 68 A.D. or so.
The letter seems to document the absolute sufficiency of the Lord’s death as payment in full for man’s sins to the large community of Jewish believers in Israel, many of whom had been priests in the Temple when it happened.
Considerable pressure was being exerted upon Jewish believers to revert to Judaism.
The conflict between Law and Grace is evident in nearly every chapter, as well as the necessity of choosing grace.
The Jewish believers were being told to go back to offering sacrifices again, why do that when the Real Lamb has been sacrificed once and for all, and is now seated at the right hand of God.
That would be comparing Jesus the True Lamb to a goat. A central element of the Levitical system was the daily sacrifice for sin.
God showed that he would no longer accept sacrifices when the Temple curtain was torn from top to bottom.
Although not signed by anyone, I believe this is letter was written by Paul. Surely most of the Jews thought he was a traitor. And wanted him dead. Maybe the Lord said keep it on the down low.
The letter hinges here. Time to leave the milk,as it were.
Hebrews 6:1-3 New American Standard Bible 1995 (NASB1995)
1 Therefore leaving the elementary teaching about the Christ, let us press on to maturity, not laying again a foundation of repentance from dead works and of faith toward God,
2 of instruction about washings and laying on of hands, and the resurrection of the dead and eternal judgment.
3 And this we will do, if God permits.
The elementary teachings about Christ to which the writer refers are those having to do with our salvation. By his examples, he shows that what he’s about to say does not apply to the salvation experience, which includes repentance, faith, baptism and such, but things that are important to more mature believers
. So what does come after salvation? Walking in victory and enjoying the blessings that come from being in fellowship with God.
Now on to the meat.
4 It is impossible for those who have once been enlightened, who have tasted the heavenly gift, who have shared in the Holy Spirit,
5 who have tasted the goodness of the word of God and the powers of the coming age
6 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.