Heb.6:4-6 & Scripture as a whole - nowhere - teach "if you abandon Christ you abandon that atonement and you can do that only once according to Hebrews 6:4-6".
6 and then have fallen away, it is impossible to renew them again to repentance, [d]since they again crucify to themselves the Son of God and put Him to open shame. (Heb.6:6, NASB)
[d] while
Bible Gateway passage: Hebrews 6 - New American Standard Bible
"That Ominous “Impossible”
What is the plight of one who has known and experienced the lovely blessings detailed in Heb. 6:4-5, and then he defects? What is his spiritual status should he “fall away”? And “fall away” from what? From the grace that had been bestowed upon them (see Heb. 12:15; cf. Gal. 5:4).
The answer to the question is this. “It is impossible to renew them again unto repentance.” The phrase is frightening. It sounds so bleak. It is bleak! It is an “impossible” endeavor.
The word “impossible” derives from the Greek adunatos, literally “without power.” Note the word’s use elsewhere in this letter (Heb. 6:18; 10:4; 11:6). Any attempt to weaken the term is an exercise in futility.
But, as our original question suggested, the passage does seem to conflict with so many other biblical texts that lavishly describe the unfathomable love and mercy of our wonderful and benevolent Maker.
How is one to reconcile this seeming difficulty?
The key to the solution is to be found in the following phrase. The American Standard Version renders it as follows:
“seeing they crucify to themselves the Son of God afresh, and put him to an open shame.”
The ASV footnote has it: “the while they crucify....” The English Standard Version reads like this: “since they are crucifying once again....”
The translators are attempting to give the English reader a hint as to the force of the original text. Here are some important grammatical facts.
They keep on crucifying Jesus
Both of the expressions “crucify” and “put him to open shame” are present tense participles. In Greek, the present tense has more to do with the type of action, rather than time (the latter being secondary). The present tense represents an action that is in progress, and generally, one that is sustained (see Wallace, 518).
The “crucifying” and “putting to open shame,” therefore, represent on-going actions on the part of apostates.
As long as ...
It is also important to note that present participles normally express action that is contemporary with that of the main verb of the sentence (Wallace, 625), which, in this case is “renew.”
In other words, “while they continue to crucify,” “as long as they are crucifying,” etc., the Son of God, they cannot be brought to repentance.
Why is this the case? Because Christ is the motive for repentance! How could one possibly repent of falling away from the Christian faith, if he believes that the crucifixion of Jesus was a just sentence upon a false Messiah?
As F. F. Bruce expressed it: “Those who repudiate the salvation procured by Christ will find none anywhere else” (149).
Conclusion
And so, it is not the case that Jewish Christians who abandon the faith cannot ever be saved. The tragic reality is this. They cannot be saved if they drift into a state of unbelief and remain that way!
But, as Blackwelder observes, the temporal participles imply that “if persons guilty of such sin will cease it, and repent, they can be reclaimed” (104).
While the passage contains a fearful warning for apostates, it does not suggest a state of utter despair."
Are Apostates from the Faith beyond Repentance?
-------------------------------------------------------
https://www.tentmaker.org/books/hope_beyond_hell.pdf
Unique Proof For Christian, Biblical Universalism