This is St. Paul speaking to Jews who converted to Christianity and are now having second thoughts. Reading only those isolated versus, it makes it sound as though there is no such thing as reconciliation, but that is not what St. Paul is talking about. He is saying to them that if they now reject Christianity and return again to Judaism while thinking nothing has changed from the old ways, they would be effectively crucifying Jesus (i.e.- rejecting Him) all over again. If they were of a mind to do that knowing full what what His teachings are, the chance of them changing their minds AGAIN and coming back are absolute zero.
This is right after (in Ch. 5) he explains Christ’s priestly ministry (our salvation), citing the words of Psalm 110, saying Christ was, after having been made perfect, “named of God a high priest after the order of Melchizedek.” Melchizedek was King of Salem (Jerusalem) and came out to greet Abram upon his return from rescuing Lot (Gen 14:18) and offered bread and wine in celebration of Abram’s deliverance from his enemies. So St. Paul establishes for them Christ’s role as high priest of His church because some of these Jewish converts began to quibble over how Jesus could be a priest at all since He was of the tribe of David and not of the tribe of Levi or a descendant of Aaron (the tribal source for priests among Jews). By appealing to David’s words in the Psalms, St. Paul is demonstrating that there can be such things as priests outside of the Levitical line, as Melchizedek certainly preceded the birth of Levi by 3 generations.
Then St. Paul gets on them for having idle faith, saying they are still like babes when they should be, by now, like men.
Then comes Ch. 6
So in answer to your questions directly:
Who can this pertain to?
Who should this pertain to?
Who does it pertain to and why?
It directly pertains to the Jewish converts St. Paul is talking to who are sort of going through the basic motions but not really trying to grow their faith. The lesson in it is one for every believer. The lesson about idle faith or turning back to another faith after hearing the Truth pertains to anyone who believes in the Church founded by Jesus Christ.
Or are they just words in a book?
Hardly. They are the word of God (in a book)